When applying a bevel modifier with an even segment count the original seam edge remains. This edge should be considered the new seam but instead we get a seam that is completely wrong.
In addition when applying the bevel modifier it will break the existing UV unwrap.
For example, this set of UVs:
When a bevel modifier is applied to give support loops will result in these UVs:
Which may seem fine at first glance but zoom in and you will start to find little tears and breaks in the island:
Alternatively we could perform a seams from islands to really see our new UV seams.
Here is an illustration that shows some of the issues we get.
This may seem like small problems and an easy fix but when creating meshes ten or hundreds of times more complex than this the time required to fix these issues multiplies too.