I was specifically referring to the type that affects core gameplay. I cited the moves Nature Power and Secret Power, both of which change depending on terrain (but that's all terrain would affect). It would be neat to actually be able to use those moves to their full effect in competitive (or at least semi-competitive) matches.It depends. If it's just an aesthetic difference between terrains, that's cool, but once it starts affecting the core gameplay (such as weather or even the move Misty Terrain) it starts to affect balance, and that's why it's necessary to ban stages that have that kind of effect.
I feel like this applies equally to items. A lucky item spawn is an RNG that gives one player a slight advantage regardless of skill. In games like Halo, weapons spawn at specific points at specific times, which means skilled players can take advantage of them. If smash bros items were more predictable, I wouldn't have an issue with them competitively, but as it stands, they interfere with determining who is the better player.
On a Smash Bros related note, it's not really the same at all. It'd be much like stage selection rather than items in that you, the players, before the match starts, could choose terrain types. Items appear completely at random, whereas the effects of the stage (or terrain in Pokémon) are much more predictable. You know what Battlefield looks like at base, just like you would know that a grass field turns Nature Power into Energy Ball and makes Secret Power inflict Sleep. An arena where there's a random chance of rain or intense sun on a turn-by-turn basis would be much more like items in Smash in terms of how it affects gameplay.