Ok, so unlike most people here, I play Flashlight mod full time, and played Hidden full time before that, so I'm qualified to actually talk about it.
Vision mods are not meant as a way to improve yourself. Hardrock and random can be useful training tools, but vision mods are not useful like that. Vision mods are a personal preference mod, not something that is useful for training.
One thing vision mods have in common is that they all reduce the amount of stuff on screen at once. At first this doesn't sound useful, especially since increasing your speed also reduces the amount of notes on screen. But vision mods do have their own uses. For example, you might not be able to raise your scroll speed high enough to reduce the amount of notes on screen at once enough to be comfortable without ending up scrolling too fast to read. Vision mods allow you to keep a reasonable scroll speed and still reduce the amount of notes on screen quite a bit.
Flashlight's biggest bonus over Hidden and Fadein is that it reduces the amount of notes on screen, AND it doesn't change. Both Hidden and Fadein change with combo, but Flashlight does not, which gives it some advantage over both of those mods.
The biggest disadvantage of vision mods in general is speed changes. Hidden only covers the bottom, so speedups are not as much of a problem as slowdowns are, and Fadein only covers the top of the screen, meaning slowdowns are not as much a problem, but speedups can be worse. Flashlight on the other hand suffers from both kinds of speed changes.
When I switched from playing Hidden to playing Flashlight, I reduced my speed from 27 fixed to 13 fixed, so about half the scroll speed. (Ok, that's a bit of a lie, fixed speeds did not exist when I played hidden, but the bpm scaled speeds I used were equal to about 27 fixed)