The reason VFD (line interactive) UPSs can still upset some connected devices is that until there is an under or over voltage condition, the incoming mains supply is simply fed through to the load. This means that the UPS does not reduce the efficiency of your data center. Unfortunately when an under or over voltage event does occur, big high current relays switch the AC outputs from the incoming mains line to the output transformer which generates a nice clean 50/60Hz AC power signal. The relays can take varying amounts of time to switch over and settle (10-50ms = several mains AC cycles). Therefore your connected hardware must have large enough reservoir capacitors to sustain a steady DC level inside your equipment even in the event of brown-out conditions.

The always online double conversion UPSs do not suffer this problem as they are always converting from AC to DC to AC (with the battery in between).

As is true with most things in life - you get what you pay for!