I've had no trouble using Cellma, asides from the random £14 invoice that appeared the second I pressed the "class 1 application" button by mistake, and which I can't seem to cancel... I use Chrome/Edge on a Windows 10 PC, and Chrome on an Android mobile.
Regardless of the new system, the CAA seem to be pretty slow and inefficient at the moment... My AME sent in my referral over three weeks ago and I still haven't been contacted by an assessor!