A folder is just a specific type of send, and should behave as such in all ways. However, when CPU-heavy FX are placed on a folder track with many child tracks this can result in stuttering and crackling, whereas the exact same setup but routed using sends instead of folders is fine.
See this thread for a little more info:
https://forum.cockos.com/showthread.php?t=220224
UMC204HD, Windows 10, latest reaper (5.978)