Normal suppressors and normal ammo - spotted at 40 meters or less.
Heavy suppressors and normal ammo - spotted at 20 meters or less.
Normal suppressors and subsonic ammo - spotted at 20 meters or less.
Heavy suppressors and subsonic ammo - no spotting, no directional hit indicator.
The RM-68 is the only gun with a suppressor as its best muzzle attachment, the shortened barrel attachment for it is suppressed but also increases ROF (besides the BSV-M since it only has suppressed barrels). There's not really much reason to use them now besides that, but the wrapped suppressor can be useful on some specific guns that lack any other attachments (although factory barrel would statistically still be better). Suppressors do disable muzzle flash so it can reduce your visual signature though, which can make it difficult for people to see where you are exactly. The main issue is that to get an appreciable advantage with a suppressor, you also need to run subsonics which is essentially two negatives for a single positive effect. On top of the issue that subsonics aren't available on every gun with a suppressor, and they also reduce projectile velocity, rate of fire, and typically come in very low capacities.
Basically, the BSV-M is the only gun in the entire game that actually gets a significant advantage from running subsonics because every single barrel on it is already suppressed AND it has subsonic high powers which is the best subsonic ammo.