Well, going through them:
Fighter: Barbarian and Kensai are both pretty awful in terms of AC, and AC is essential in BG1. Wizard Slayer is pretty useless all the time anyway. Berserker is GOOD, but not being able to specialize in ranged weapons is a relevant downside in BG1 where ranged weapons are incredibly dominating for most of the game and arguably one of the best specializations for a fighter. On top of that Berserker is only getting 1/2 rages a day, so unless you really sleep a lot in those outdoor areas it's not commonly a buff in play. It's a good class but not necessarily overpowered. Would rather be a fighter/cleric. Arguably a berserker->cleric dual is a bit overpowered I guess.
Ranger: Archer is great, but doesn't even get bonuses until level 3 and again at level 6. Possibly a little broken. Stalker has awful armor in exchange for its perks, I say it's a net loss overall. Beast Master has awful armor too without even getting any relevant advantages. Would rather be a ranger/cleric.
Paladins: Basically a 100% worse fighter in every way in BG1, so a slightly better version of them is fair. Remove fear is a level 1 cleric spell that lasts an hour, who actually cares. Inquisitor has no real casters worth dispelling without SCS. Undead hunter is great, but +3 to hit and damage vs. undead is fairly comparable to the fighter getting +2/+3 vs everything from high master in their weapon (granted undead hunter gets it at level 1 while fighter has to level a bit). Black guard is OK I guess. All of them are objectively better than the paladin but fairly comparable to a straight fighter IMO (A dwarf fighter is strictly superior to all of them I think since they'll get 19 con and +5 con bonus to all saves, paladins can't be dwarfs). Also inferior to a fighter/cleric.
Clerics kits are... OK I guess. Inferior to being a fighter/cleric.
Druid: Shapeshifter is not immune to normal weapons, just tested. Also tested totemic druid and the spirit summoning ability summons absolutely nothing on my 100% vanilla, updated BG1EE, so lol beamdog bugs. Avenger is OK I guess. Would rather be a fighter/druid. In fact Cleric/Ranger in BG1 non-EE is objectively stronger than fighter/druid since it has all druid and cleric spells, so you're actually being nerfed here while playing the EE (unless you edit the config files to undo this change).
Mage: Wild Mage is basically suicidal. Sorcerer is OK, don't know if I'd call it that overpowered unless you are powerleveling basilisks early in order to get spells that you'd only find in the city.
Thief: All of the kits really suck and are absolutely inferior to fighter/thief, especially at low levels. Bounty Hunter, Assassin and Shadow Dancer literally can't do what they are designed to do because it doesn't unlock until later (Assassin) or they don't have the skill points because they need find traps first and they get skill points so slow (bounter hunter, shadow dancer). And Swashbuckler is a joke class for people who want to be a fighter/thief without knowing fighter/thief exists.
Bard: The kits are definitely all stronger. Jester's song will hit all the time at low levels, Skald's +2/+2 bonus applied to everyone makes the berserker's one person limited time rage look wimpy, Blade gets its abilities way too fast IMO. There's no good multi or dual that really replicates this and these are all unique and useful things. Could see an argument that they are overpowered but I don't think they trivialize the game. Maybe jester does though, I don't know exactly how awful mid to late game enemy saves are.
Monk: lmao hahahaha
Shaman: see monk
TL;DR while some of the kits are improvements most of them still pale in comparison to a multi, so unless you got a very specific comparable dual you're behind and even with a dual I don't think you are significantly stronger (fighter duals suffer because they don't get grandmastery until BG2 which is kind of the point of dualling fighter). I keep mentioning fighter/cleric because it is just that good, especially if you're a dwarf.
Also, it should be noted that BG1EE necessarily nerfs most melee classes compared to BG1 due to the fact that it splits all the proficiencies BG2-style, which means that there are much longer gaps between weapon upgrades than the original depending on what you pick. Fighter/Cleric for example literally only had to put 2 points in spiked weapons and blunt weapons to have full effectiveness with every cleric-available weapon in BG1, in the EE there's way more options.