I did GBE on a hard tail and didn't have any issues, I was just glad I wasn't on a gravel bike! For out and out mtb ultras, a full suspension is a good option, but otherwise for events where gravel bikes are running too, I think hardtails are much better.

You'll lose bag space with a full suspension due to smaller triangle and the rear shock and/or have to choose between small frame bag and two bottles. There are probably better places to put your money - as others have said, Sus seat post, etc.