Well, I'd say you need to distinguish sets. Like, .96 and .96 deco are not the same, aero MG and aero PG are not the same, and so on.
First thing I want to say is, some weapons are underestimated just because they're not meant to be a killer. If you want to play in solo, it's kinda true that having an easy-killing weapon is better than a more support or turf oriented one (it's not 100% true, but well I'll still put it this way). And some players don't take it into account. Like, the weapon is not good to kill, then it's a bad weapon. Which I feel is not true.
For example, I played twice against an inkstrike brush. He won twice. He was very dedicated to paint the whole map in his color, and he was winning this way. His ratio had very low numbers, like 1-4 in a 5 min game, but his plan was working. We were on kelp dome, and I swear by the middle of the game, both trees and center were all his color. Sure we took the lead in the first part, but in both games his teams could win in the latter part. He truly found a way to play his weapon.
So I would say if you're going to talk about bad weapons, you need to understand what the weapons is meant to do and say why it's not good at it (why others are better for instance).
Personnally, I feel the sloshing machine is not good. From what I can see it's not meant to turf, but it doesn't kill well either. It's not that good at supporting either. I'd say it's meant to make weird kills, but these are not common enough. So in most situations, you would do better as a support with more range, with burst bombs, or you'd be better with a blaster. I think a longer range would be quite interesting.
I would say the brush with mine/shield is not good either. It has absolutely no range, the shield can be pushed because of that, and even when in range it's kinda long to kill. So it's not a killing type. As a support, no range at all won't make it, only the shield. As a turfing one, you have better options.
The rapid blasters are said to be bad because they are not easy to learn, and you need to be good at finding and keeping the right distance. Sometimes you come across a good player on them and you understand they're annoying. I'm playing the two rapid ones, and the rapid pro deco myself, and I would say they're from ok to good. The pro vanilla should be ok, tthough I believe hit has one of the weakest seekers as the weapon won't charge in anyway.
About H-3, I would say the sensor-zooka one is not great, when compared to forge pro. It *has* some good points (turf coverage or ink consumption I guess), but overall I feel you can do most of the job with the forge, while you can't move the same way with the H-3. The other one is questionable as the subs are unique I believe. A little more range or a faster firerate would be better I believe. Though I would recommend using these weapons as a learning tool.
So that's my feeling about these weapons. As they are now, I would say the sloshing machine and the H3 need too much accuracy. I think squiffers are questionnable, but it might be my weakest weapons, so I can't really talk. I'm not great with chargers, so not having the range to survive is too much for me.
Edit: but there is no weapon bad enough that you can't go S+ with from my point of view.