We have used it a lot but just with a 4m SIB. The T-end is only for working and trip boats plus club launches etc. The main finger is where we go but sometimes there isn't enough room to be alongside you need to tie up nose on. There are also a good few local working boats with a lot of sticky out bits when you have tubes to consider.
Another hazard is the crabbing families who are oh so reluctant to lose their spot to a mere boat which wants to tie up.
As the tide drops only the end of the pontoon is afloat so it really concentrates the craft onto a small area.
Having said all that I like Mersea and we always manage to squeeze in somewhere.
For your RIB it may be worth phoning the Yacht Club re a short duration visitor mooring and checking their launch will run for a visitor then you will know you can radio on the day and get sorted... the launch was much restricted in the peak of Covid times and I'm not sure if they are permanent members only now. If the launch won't run you can often hitch a ride for a few pounds with Stacey on her Lady Grace small open trip boat which runs frequently at casual times (she has a website with info).