The underage rental fee is only $15 /day. They only hold your reservation for two hours. After that they reserve the right to cancel your reservation and/or change your rate. Only an issue if your flight is delayed. Major agencies will generally hold your reservation and rate until midnight and will almost always hold your reservation if the flight you list on your reservation is delayed.
I doubt ACE is a large enough player to get many (any) reviews on DIS (good or bad). There are enough codes going around so people can generally get an acceptable rate with one of the major agencies.
Lower rates combined with the $15/day underage fee suggests this may be an agency worth considering for renters age 21-24.