The reason these limits exist though for projectiles and monsters is that they can be costly to calculate, and Myth does most things in linear time, so every tick it goes through each projectile one-at-a-time and processes it. This is done without many of the optimised constructs modern games use today, or without various (complicated) techniques to combine groups of similar type to process them more quickly.
For example; when a projectile is passing over a cell that contains a model, it will process every polygon in that model to see if a collision occurs, without trying to eliminate any that it can't possibly hit first. Myth isn't designed to have lots of models though, so this isn't really a big deal; if it had been then I expect Bungie would have optimised it further.
It also increases the memory requirements, as Myth will allocate enough space to track the full number of possible entries. It doesn't allocate the full amount of memory needed, but bumping a limit will immediately increase minimum memory requirements. Additionally, increasing a limit may adversely affect parts of the code that may rely on a particular limit existing, or use some tricksy logic to optimise something in a way that just doesn't work if the limit is increased.
So increasing the limits will almost always make things work worse for lower-end players, who still need to be supported to some extent as Myth is after all an older game
