The Druid ones, for example, are pretty bad.
Horrendously wrong. Totemic Druid in default BGT/EasyTuTu didn't have a proper level 1 version coded in of their pet, so you absolutely trivialized the entire game by tossing in a tanky summon with immunity to normal weapons. Avenger has three very interesting forms with good application and nullifies the Druidic problem of having a virtually dead 2nd spell level by simply spamming Web and being immune to being held in your own web in the Spider form. Even the much-maligned Shapeshifter gets to at least utilize the werewolf form for a non-trivial boost to a few stats in the early game. All the Druid kits are insane in BG1 and Totemic/Avenger bring in a ton of power into early SoA at virtually no downside. Even in the more modern installations where the scaling of these abilities has been fixed, Totemic is still bonkers in BG1. You're still likely better off being some form of Fighter>Druid, but you're better off being a dual for half the builds in the game anyway, so that point is moot.
The Ranger ones (other than the Archer) are questionable.
Beastmasters aren't a particularly hot kit, but in BG1 having a guy with a bow who spawns their own summons and can abuse the perks of Find Familiar is actually very strong. There's also nothing wrong at all with Stalkers; the "no metal armor" tradeoff is not that big of a deal, and it does encourage the player to actually use stealth, which is an underrated mechanic for non-Rogue classes.
Rogues are also questionable.
Assassins can apply Poison Weapon to any ranged weapons (most notably Darts because at 3 APR you have the most attempts to poison someone in a round) for instant trivialization of fights with casters (which can still be troublesome in BG1), and it passes through Stoneskin which SCS mages are fond of, for example. Bounty Hunters' Special Snares are a good source of crowd control and they're thrown from a distance, and their overall package scales well into BG2. There's also nothing wrong with Swashbucklers, although they're better off in BG2.
The only two kits that are plain worse than the base class are Beastmaster and Wizard Slayer, with the former only really sucking in BG2 and the latter at least has some entertaining cheeses like applying Miscast Magic to the projectiles conjured by Fire Seeds (including their AoE) or Melf's Minute Meteors, facilitating some interesting duals. Shapeshifter is somewhat of a grey area because their malus is miniscule (Druids don't really care about armor that much anyway) and popular mods throw them a bone. A few of the Mage specializations (technically not kits) possibly count, but I'd honestly figure that the only truly debilitating specializations that offer no real bonus while carrying hefty tradeoffs are Diviner, Transmuter and Abjurer.
There are many more kits that are a straight upgrade to their base class (at least one per class, actually) than there are utterly bad kits, and I'd wager there are comparatively few kits that achieve a healthy balance of numerous tradeoffs.