I'm a crotchety old fossil who would normally find something to complain about for everything, but in the case of BIONICLE sets I tend to find something to love about all of them. That said, if you asked me what my least favorite set(s) were (which is basically what this topic is for) I could give you a handful of sets I had the most minor gripes with.
I gotta admit the Exo-Toa was a pretty cool set, but as Onua is my favorite Toa, you can probably imagine my disappointment when I found out he couldn't actually fit in the thing (without, y'know, rebuilding him). Pohatu can't fit in it either, meaning only four of the six Toa could actually fit in this thing. (Or nine out of thirteen, if you count the Toa, Toa Nuva, and Takanuva all together.) Also, one of the gears on the back kept falling off, which was annoying. Pretty fun set though. Liked the punching hand.
Speaking of Toa Nuva, as someone who witnessed firsthand the Great Mistika Quarrel of 2008, I'm surprised no one's mentioned them or the Phantoka yet. (Or maybe someone did. I kinda skimmed through this topic.) I don't hate then the way people did twelve or so years ago, but overall they were pretty meh. The Makuta Phantoka were okay at best, but I found the Makuta Mistika to be pretty messy builds. As for the Toa... well, I've said it before and I'll say it again: I'm pretty sure they designed some generic Toa sets beforehand and slapped the Toa Nuva's names on them at the last minute. Obviously, with Onua being my favorite Toa, I found Onua Mistika to be the biggest offense: the big shield and heavy-looking launcher were plenty Onuaworthy, but the lanky, stick-thin proportions were not. To add insult to injury, prototype images of Pohatu Phantoka used black parts, and actually for made a better Onua. (I kinda liked the Toa's masks for some reason, though.)
Now, if you asked me what my #1 least favorite set was, that would be Roodaka. Between the, erm, anatomy, and the high heels, you've got a set that's trying a little too hard to be a female character. (Not to mention, though I have never personally experienced this, I have heard that high heels are insufferable to wear for long periods of time, so I assume Roodaka exists in a state of constant pain.) I skipped out on Roodaka back in 2005 just because I felt like owning that set would be kinda awkward. I eventually did get two sets years later, but that was just so I could build the Shadowed One and Voporak. Anyway, now I have four silver Rahkshi heads, which were the best part of the set. You wanna build silver Chain Lightning Rahkshi? So do I. They're great parts. That's the best part of this set.
anyway that's my essay