Honestly I'm not an expert on that part of the lua internals so I can't confidently say why. I don't think it's a fundamental problem since later versions use the same approach afaik, so I assume it's just implementation nuance. 5.0 used, IIRC, mark-and-sweep which is not great either but it's predictable: it's easy to develop intuition for how much you're allocating and what the collection cost will feel like.

The main practical issues I've run into with 5.1:

  - per-object overhead in time and space both increased significantly, making many small allocations much more expensive than fewer big ones. further pushing you into the "mutate a few huge global tables" pattern that is already a problem in lua conventions.

  - some of lua's more lauded and genuinely powerful features, esp metatables, don't work smoothly or predictably with the GC. it leads to people trying to manually tune it with __gc metamethods, which is hard to do correctly and causes memory leaks if done incorrectly.

  - weak tables idk,. esp with strings for some reason. similar to the __gc, there is a solution there but in practice it's very hard to be sure you've got it right and it's worse to get it wrong than to not try.

I'm sure a PL expert can explain and justify all this but I am just a normal person who codes for money. Lua is admired for being simple and accessible, but especially in the older versions you get into the low level weeds real fast over this stuff. Kind of a good example of minimal implementation exposing you directly to irrelevant complexity, really.