As a holster maker, one of the problems I've encountered is that IWB holsters will shift. The shifting causes the presentation of the gun to change. It can annoy the user and result in unconscious adjustments that show the observer the user is carrying a weapon. The causes of rollback include: pressure on the holster from the pants, angling of the belt has the hips move back while sitting, weight of the gun, loop-belt interface, and loop location. The angle of the hips determine the extent of rollback when sitting.
One way to mitigate "roll back" or "set back" is to have some sort of wing that presses against the body.
S&W 625 IWB holster with wing and non-inline widely spaced loops
The wing presses against the body when the loops fail to resist the weight of the gun. It does help a bit when seated, but it's more to support the loops. It also helps shift the gun up when pressure from the pants presses against the muzzle end of the holster.
Split loops provide a better base of support for the holster while assisting in concealability by reducing overall thickness. However, any holster attached to the belt will tilt with the hips. Only posture, holster placement and forward cant can mitigate this problem. I move my holster farther forward (8:30-9 o'clock/3:30-3 o'clock) and buy only holsters with extreme forward cant. Sitting up straight when seated does the most to prevent printing from rollback.
The belt loops of the holster need to be matched to the width of the belt. No holster in the world will present the gun consistently if the fit is loose. Some manufacturers, seem to have over-sized loops on some models. For example, the DeSantis Cozy Partner comes with 1.5" loops that fit up to a 1 3/4" belt. I replace them with spare loops from a Milt Sparks Heritage holster. Blade-Tech rubber loops can also be large. Adjust them appropriately. A proper fit of loops to belt will help stabilize the gun.
One problem I have noticed is that closely placed loops will exert pressure on the belt, causing it to twist back if the gun is heavy. Combine that with hip movement and a leather or nylon belt will weaken over time. A steel framed gun will enhance the problem (polymer has a distinct advantage in this area). It is for this reason I'm testing biothane belts, which seem to resist this problem a bit more.
Preventing rollback is important for IWB holsters. Forward cant and gun placement are the primary means of preventing rollback. Some holsters include a wing to stabilize the gun. Widely spaced loops provide better support and a stout belt will resist bending for a longer period of time. Sitting also causes this problem and the angle of the hips that determine the extent of the rollback.