the use of 96KHz is recommended when digital domain recording at such sample rate is necessary. For any other app., like instrumental use, 48 KHz is the best sampling rate. Higher sampling rates cut DSPs resources in half so you can have some limitations.
As adviced in private corrispondence w/Eventide Support, we have no problems with 6620 Monster Rack at 96KHz. If you are using external clock, it must be very, very stable though, to run large presets (monolithic) as this one.
all the best