A few things. For starters, there's allowed to be some overlap between the sites. When deciding scope we need to ask ourselves: Is this the kind of question we're qualified to handle? Is this the kind of question we want people to ask here? If the answer to those questions is no, then we should ask ourselves "Is there another Stack Exchange site we could redirect these questions to instead?". The key thing is that the scope of other Stack Exchange sites doesn't matter to us when deciding our scope. If we do decide something is off-topic, however, then having another Stack Exchange site to redirect them to is a good idea: it helps our sister sites grow and helps the user get their problem solved. Win-win!
Next up, as Apaul already said, there is a difference between a professional solution and a hack. Professionals will have tools and skills the average person won't have access to, so the solution which works for a professional won't necessarily work for the average Joe (or Jill). So even though the professional solution might be the best solution to the problem, it may not be one Mr. Average Joe is able to implement, and so isn't that useful to him. A Lifehack, on the other hand, could be something Mr. Average Joe could implement on his own. So although it might not be the ideal solution, it could be the only thing that works for him (without spending a bunch of money hiring professional, anyways).
Now, with those two things out of the thing, let me be perfectly blunt: Lifehacks is going to attract lower quality answers than more specialized sites with a more well defined scope and expertise.
Here's the thing, as much as I know there are a people passionate about this site and it's topic space, you just can't be an expert in Lifehacks. It just doesn't make any sense. Whereas a Carpenter has a well defined set of expected skills to have, a "Lifehacker" doesn't. What this means is that Lifehacks is a horizontal site, rather than a vertical one, like most other Stack Exchange sites (Skeptics being the only possible exception I can think of). Whereas sites like Wordpress or DIY has a narrow scope with a specialized set of experts, Lifehacks...well, doesn't. Our site has several sub-categories in it since we are (despite what some people want) basically a catch all site right now. Some of the questions we face are best answered by a Carpenter, others by an Electrician, some by a Doctor, et cetera. No matter what solution Mr. Average Joe is going to come up with to How can I clear my shower drain without drain cleaner?, a plumber is going to be able to come up with an equal or better solution using the same set of restrictions.
Now, that might not sound too bad at first. Sure, we might have a broad scope, but that just means we'll have a larger collection of experts, right? Well, no. One of the primary principals behind the Stack Exchange model is that experts will come to sites filled with other experts in same field.
Experts aren't going to trust the opinion of Mr. Average Joe on how to best solve their problem, they are only going to trust the opinion of a fellow expert. After all, if an expert can't solve their problem, why would they expect someone who isn't an expert in their field to? I'm a developer by trade, and if I was running into a problem I couldn't solve, I'd ask a fellow developer for help, not a Doctor, just like a Doctor wouldn't ask me for help performing surgery. A Doctors expertise isn't programming, and my expertise isn't Doctoring (which I've just decided is a word), so we're not going to consult each other for our expert problems.
A site filled with a well defined set of experts is also going to provide verification which a broad site won't. If I visit a site filled with Plumbers with my plumbing problem, then I know that if someone posts an incorrect solution that will be downvoted and someone will point out why it's wrong. When someone comes along with a solution which works, it will get upvoted. This verification means that I can trust the site to give me a solution which works, and won't blow up in my face.
Something else is where an expert expects to find other experts in their field. A Plumber isn't going to stroll into Wordpress.SE and expect it to be filled with fellow plumbers, they'd expect it to be filled with Wordpress developers. Similarly, that plumber isn't going to come to Lifehacks for their plumbing problem because they wouldn't expect it to be filled Plumbers. Even if they expected to find a bunch of plumbers there, they'd also expect to find a bunch of non-plumbers (Mr. Average Joe), whose opinion they don't really care about, and could only serve to bog down the problem solving process. Which is a good segway into my next point...
A site with a well defined and narrow set of experts is going to be provide more consistent and quicker results than a site filled with a bunch of experts from different fields.
Unlike our hypothetical plumbing site from above, a broad site like Lifehacks doesn't provide expert verification for it's solutions. Even if Lifehacks is filled with plumbers, they're not going to outnumber all Mr. Average Joe's on the site, who might pop in to upvote incorrect answers because it just looks like it might work, in their non-expert opinion. So that plumber can no longer trust that the answers posted are being correctly verified by fellow experts, and so he can't really trust the site to solve his problem, so he'll go elsewhere.
A site with a more narrow scope also ensures our plumber that his question is going to be seen by his fellow plumbers and answered, not lost in a bunch of questions about surgery, gaming, wordpress, carpentry or any other number of subjects. If the question gets lost amongst all these other questions there's a good chance it'll go unanswered. This is why a bunch of sites which had overlapping with Stack Overflow started their own sites (i.e. Wordpress, Drupal, Server Fault, et cetera). Stack Overflow is huge, and it's really easy for your question to get lost in the huge flood of questions. Due to this, if I have a Wordpress problem, I ask over on Wordpress.SE for a solution. Not only is it less likely to get lost due to the lower question volume, but I know that when it is seen, it's going to be seen by the people most qualified to answer my question, as opposed Mr. Average Joe. We're obviously never going to reach the same volume as Stack Overflow, whose sheer size is the cause of many of it's problems, but it should make you stop and think. If Stack Overflow, a site with a much narrow and better well defined scope than ours is having this problem, then won't it be a lot worse for our Lifehacks sites, with it's much broader and less well define scope?
Now I've only talked about experts, but all this applies to non-experts to,in fact it applies to them even more. A non-expert needs to be put their trust completely in the site itself, and the people in it. If someone posts an incorrect solution to a question put forth by an expert, then there expert is likely to realize that. A non-expert, however, our Mr Average Joe, has no chance of determining that for themselves. He needs to be able to trust the people on the site are experts, and that there are experts verifying everything that's posted. If he can't, then he's unlikely to come to us for help.
This post is...much longer than I expected, and it just kind of got away from me, and I definitely veered outside the scope of the meta post, but answer quality is such a vital and important thing for any SE site that I just had a lot to say. The thing is, I'm really doubtful this site can work, which anyone whose engaged with me in chat or meta is probably aware of. We have a lot of problems, and I think answer quality and not being able to attract experts is one of our most major and fundamental problems. One, quite honestly, I don't think we can solve. Let me close this off before I veer even more off-topic, but let me just say this: If our site isn't going to help both the experts and Mr Average Joe, then who is the site really for?