Some may consider this unprofessional (I guess it is, like, literally) but I firmly believe that, at the end of the day, you don't owe anyone even the smallest amount of misery if you're performing a service for free.
Like, when I really don't want to draw a commission, even if I'm super tired and I'd rather be stabbed through the eye, I still power through, because someone else gave up something of value (i.e. money) with the expectation that I would deliver something on a certain date. It's a transaction-- they gave something to me, so I have to give something back to them.
Comics, however, I've always drawn completely for free. Often I don't even get comments, let alone engagement or money. So if I find I no longer enjoy working on a series or want some time away from it...there's no sense of reciprocal responsibility that might make me 'power through' anyway. No one's giving anything to me; why do I have to give something to them? As they say, "if it sucks, hit da bricks".
That was essentially the reasoning behind DotPQ's year-long hiatus, and even now that I've brought it back it's been 4 months since the last update. ^^; Although people barely even seemed to care when it came back from its normal, properly scheduled hiatus between Season 1 and 2, so it's hard to feel guilty. I remember working all night to try to get Season 2's first episode out on time...little did I know, Season 1's momentum was already gone for good, and it would all be for nothing. =T
I've put things on hiatus for any number of reasons, even if it's as simple as "I got other things to do". And I think all those reasons, or any reason, is valid. It's your life and your time, you shouldn't need the approval of strangers to use it how you want.
Generally this doesn't impact my projects any more than the whims of fate, as I explained above...at least, externally. Internally, it usually just gives me time to think about the project outside of other people's influence: how I truly feel about it, how I might want to improve it, or whether I think it really needs to continue.
That's when I think a hiatus is most valuable, when you take that time not only to rest, but to prepare yourself to change something after that rest period ends. If it wasn't for that one-year hiatus, I wouldn't have taken the time to refocus on my animation and video editing skills, and I wouldn't be currently enjoying methods of storytelling that other people actually seem to be interested in. ^^ RIP to my 1.5 webcomic readers, I guess, but I'm having fun creating again, and in the end that's all that matters.