If use PU or fibre bodykit.....can be removed easier when bored with the bodykit...
If use custom sheet metal as bodykit (welded to car body), can be very tricky to remove from the car later.
Most of the time, fibre body kit requires lesser filler to cover imperfection, while custom sheet metal bodykit need more filler (also depend on sifu skill). Sometimes, if done inappropriately, the sheet metal would rust under the paintjob. In a few years time, the filler will crack and kopek from the sheet metal due to rust underneath......
Anyway, these are just my opinion....