So, I'm very familiar with the New Vegas mod limit... textures and whole models seem to vanish from the world.  What happens with Fallout 4 when it reaches a certain mod level?  Same thing, textures vanished?

it either won't load anything after the 256th plugin (.esm/ .esp), or ends up borking your game hard (frequent crashes)

In NV it used to just crash and give weird textures and also didnt let you save the game. While i never went over the mod limit i suggest to not do it or you might destroy your save/game. Just stay under it and disable mods you never use at all or merge them.

I've had some weird experiences with FO4 once I went over the mod limit, I recall this one time where all NPCs would refuse to move (or even seem animated for that matter), as well as having multiple crashes and corrupt save games. All in all it's best to not surpass the mod limit as it will just end up ruining your experience.

It's also a question of scripts, some mods are particularly resources demanding and create a latency that finish to prevent scripts to execute properly. In this case, merging won't help. Well I suppose it's also the case with FO4 isn't it ? (I don't have it installed yet)