When running ubi over block2mtd over FTLed flash, FTL in flash still operates, and affects both performance and reliability (i.e. wear-leveling).
UBI guaranteed wear-leveling is effectively turned into random thing.
FTLs are known to start badly misbehave after some time if they don't get information about which blocks are free. So unless TRIM commands are used, "ubi over block2mtd over FTLed flash" will badly degrade in time.
Getting raw access to flash devices (bypassing vendor FTLs) is indeed the best possible scenario, because this gives way to open development of reliable and high-performance algorithms to manage those.