Just by reading over the UFOPaedia wiki articles on the original X-Com, including the bug and strategy articles, I just got the impression that the game actually was fairly buggy, just not usually in game breaking ways. I suspect fan patches have fixed a lot of the bugs though, like incendiary weapons dealing damage to anything on the map standing in fire when they're fired, or blaster launchers having "dud" shots if you don't put enough waypoints in their arcs, or elerium powered aerospace craft being limited to 50% range. Maybe the grenade hot potato exploit too, but at least that's not game breaking and you have to abuse it yourself.
But anyway, referring to clunkiness, I meant that the game is just a lot more complicated compared to the new ones in ways that are arguably unnecessary. Inventory and item juggling, for example, is way more complicated in the original, and the way you have to manufacture and keep track of ammo for almost everything is a headache. The way TUs work is way more complex than the later games too, though I can see some value in it over the simplicity of the two actions in later games. It looked like you have a lot more options for positioning for soldiers too, like having them kneel (or go prone?) for situational benefits that are kind of automatic in the later games.
I would say that the original is also a lot more unfair in general, since you have things like chryssalids that can run up to you from beyond visible range and zombify soldiers even if they do no damage when attacking, but I fully recognize that the later games are kind of too easy in that respect. Lots of soldiers should be dying in an alien invasion.