Yes for extreme close up work, the recessed lens element of the 90 can be a problem - more with lighting the subject than anything but that's why Rollei made the M39/40 behind the lens shutter accessory so one can mount other small optics like the schneider M-Componon's, Leica Photars, zeiss luminars, and other interesting optics like the rodenstock apo rodagon enlarging lenses. For everything else the recessed front of the 90 is pretty nice to have. The schneider 150mm apo macro bellows lens is also quite a nice optic but has less contrast than the 90, though still renders fine detail very very well.
One last thing to mention about the Rollei 90mm lens. It has a very large image circle and can support large movements when fitted on the X-Act2 (rollei's technical camera).