It's an excellent powertrain for a front engine, spring suspension chassis, but didn't really catch on with buyers in that price class. The diesel engine costs more than a gas engine but the performance advantage apparently was not enough for shoppers. Those willing to pay more for a diesel also wanted air suspension & brakes, greater GVWR, and other advantages of a diesel pusher (rear engine chassis). I'm not aware of any drawbacks to the Freightliner FRED chassis coaches like the Tiffin Allegro FRED.
It's a 300 hp Cummins ISB engine. View the chassis specs at:
https://cdn.fccchq.com/specsheets/5b7abfb0cf1da258f28db09d/2008-Tiffin-Allegro.pdf