Vanilla Baldur's Gate was a mediocre game with too low resolution and poor monster AI. With TuTu or BGT, plus the BG 1 NPC Project and Sword Coast Strategems mods it is a brilliant game.
I would disagree with you. BG 1 was splendid game at it's release and had hardly any serious bugs (I played it thru over half a dozen times back in the day with no crashes or bugs that prevented progessing with story). There was nothing inherently wrong with it and I'm pretty sure I'd enjoy it even today without any mods despise technical limitations of it's time.
Obviously this does not mean I wouldn't enjoy it more
using BG2 engine and some of the mods available for it if I replayed it today but I would hardly call it broken at the time of it's release; actually quite contrary, I'd say it was best one of the best cRPG's released at the time.
I'm not saying BG was broken, only mediocre. And the low resolution means that it, unlike BG 2 and the other IE games, hasn't aged vere well when you play it on modern, large screens.
Also, the creature AI was horrible, since calls for helps weren't implemented. So it got too easy to just shoot down enemies one at a time without their comrades reacting.
One could argue that the game is broken when you need to move your characters near enough to activate the whole enemy party to give them a fighting chance...
Bottom line: Baldur's Gate was a mediocre game which needs mods to become very good, while BG 2 and Icewind Dale are very good in their vanilla form and can be played without mods.