I buy mostly Ninjago sets, and as you can probably guess, there have been some huge winners and some decidedly meh-ers over the years.
First up is 70602 Jay’s Elemental Dragon from 2016.
Admittedly, the main wave of Skybound sets from which this one comes was generally lackluster, and I bought this because it seemed like the best at the time (and also I like Jay). Once I built it, though, I was disappointed at how square and chunky the dragon is. It’s a lightning dragon, but nothing about its body shape communicates that, in my opinion. Also, half of each wing is fixed in place, which never made sense to me and frankly makes it look goofy when you move the outer half.
There’s also 70643 Temple of Resurrection from 2018.
I was kind of biased against this set from the start on account of heavily disliking Sons of Garmadon, the season from which it originates, and honestly I wasn’t expecting too much from just looking at the pictures. But in hand it’s just… boring. Definitely the weakest of the Ninjago location sets I own. There’s not really anything to it besides the main room where the Oni Masks are and the largely empty cave underneath. The folding roof function is cool at first, but (apart from the fact that it’s pointless in-story) it’s impossible to have any minifigures standing in the main room while the roof is folded down, which just cuts the space for play in half. Functions should contribute a net increase to play value, not add some and negate some…
(Also I was salty that the panels behind the masks were stickered instead of printed–the Fire Temple and Battle for Ninjago City both had beautiful printed panels of the same size and I still don’t understand why they didn’t do the same for these. The whole set is pretty reliant on stickers, actually.)
For non-Ninjago sets, I can’t help but think of 8080 Undersea Explorer from 2010.
I got this when I was in the target audience and was disappointed even then, if that tells you anything. The rigid legs really kill it for me. Yes, they allow the mech to transform into a submarine with wheels, but really I always found both configurations odd instead of cool. The whole robot is just kind of unwieldy, and as I recall the flick-fire missile holders kept falling off. The sea serpent is good, though!
Lastly I’ve got 8189 Magma Mech, also from 2010 (gee, that was kind of a bum year!).
This is actually just a bad set. The legs fall off at the knees constantly, there isn’t enough articulation to effectively use the claw (which should be horizontal if you want to use it anyway!), and the drill/water shooter is just… there…? Young MooCowsRock got it because it was the cheapest way to get all three colors of small lava monsters, but I really should’ve just saved up or pooled my money with my brother to get 8191 Lavatraz (though that doesn’t look spectacular either). The only good things to come of this set were the lava monster, the miner minifigure, and the (exclusive?) chrome silver tread links, which I later used for the helmet on this Sauron MOC. That redeemed the Magma Mech a little, at least.