There are two bugs that I've noticed with modifiers.
1. Floor tiles are not flush with blocks/walls or modifiers. When moving from a floor tile to a block or modifier tile it results in the player falling/jittering about one pixel because the floor is slightly higher than the terrain you're moving onto. It's very noticeable when you have headbob off.
Workaround is to simply not use floors at all and instead use a layer of blocks on the ground floor - which seems to already be common practice due to floors not even functioning properly above the bottom layer.
2. Stacking multiple collision boxes too close vertically will result in the player continuously "snapping" to the next one if they approach the object diagonally. For example if you were to approach this grate diagonally (and if the diagonal corners aren't being blocked by another wall/modifier) you would end up basically climbing it as if it were a ladder/elevator.
