I think it was legitimately
good, but with lower HD, the HP division, and a d4 averaged in, those characters were still fragile despite their good AC. And if you triple-classed the problem was exacerbated because you were usually two levels behind the single classed characters instead of "just" one. Initially I thought the restriction in 2E made sense, and then through play realized how much it nerfed them and prevented the core concept from working at all unless and until you found magical elven chain mail.
Ok, this makes sense. I don't factor in the increased level limits as much when thinking about 1E vs 2E because every table I played 1E at used Unearthed Arcana, and that already introduced higher level limits and some MUCH higher ones with extraordinary ability scores.
Yes, the retroactive punch in the gut to Thieves that the BECMI Companion Set administered was really uncalled for, and as I recall the Rules Cyclopedia sadly incorporates it, maintaining the TSR tradition of boning Thieves at every opportunity.
I get what you're saying about B/X Thieves. The difference is relative because everyone else is less powerful. So, for example, them having a d4 HD in B/X isn't that much weaker than having a d6 HD in AD&D, because Fighters and Dwarves only have a d8. And yeah, no Assassins and one could multiclass with Thief, so their skills were exclusive. The fact that Thieves can use any weapon and get their rear attack bonuses with any weapon in B/X are also better than their restrictions in AD&D.
I still make Thieves tougher and more skilled in
my usual B/X house rules.