Interestingly, there are also quite a few which are very, very good for a handful of missions, but almost entirely useless outside of those. Take the Brute variant with dual heavy "frame" guns--it's a helicopter with a pair of player-manned flamethrower turrets. Complete trash for almost every situation, completely broken for farming Crimson because you can hover a couple meters above the ground and kill everything with no risk. Or the Gungnir: 150,000 damage with a range longer than the draw distance, but firing it leaves you helpless for ages because it requires 720% energy.
It becomes even better when you're playing in level-capped multiplayer, so you can't just pull your strongest stuff and steamroll--instead you have to figure out what's best for the mission you're about to do. That remains the case even at high levels; on Ranger you'll sometimes want a double Lysander build, but on Machine Squad you want to take Lysander F/Z + Stringer J9 and use the rotation to reload the Stringer for that higher burst damage, and on certain (mostly the underground ones) you'll want a level-appropriate Slug Shot or Governor because there's not enough room to retreat and too many corpses to get reliable shots out.
That, and how things change over time. Early on, especially in Inferno, explosives are god because of all the AoE. Lategame Inferno, though, everyone will hate you if you spam explosives because most people are using sniper rifles, railguns, &c. that can one-shot with little chance of friendly fire, and targets being flung all over makes it take longer to kill them.
It's honestly admirable, the more I play this, how well-designed so much of it was. Almost everything has a use on some mission/difficulty combination, and the few that don't are low-level and obviously a little jokey. Like most of the spread Ixions: they're trash in Inferno because of the damage, but on Easy/Normal/Hard they are great for stunlocking and killing whole crowds of land-based enemies. And the SIG and lower-level continuous beam snipers, even if they don't do enough damage, they have infinite stunlock and double as targeting indicators for your teammates and an easy way to clear line-of-sight by chopping down trees/fences/&c. quickly.