Well, I have got into more trouble with treeless causing problems than treed actually!
Getting the pommel insert right can be an absolute nightmare, so never again would I go for one of them.
Depending on what shape of horse you have Freemax could be an alternative, but I would think only if you have a roly poly barrell shape one. Certainly not for anything with any spine showing and even with the roly poly's they do slip. Fellow livery broke her arm VERY badly after horse spooked in the arena and saddle slipped and she came off with a Freemax.
I had a Freeform and it slipped. Not so as I fell off - it stayed put for spooking (which Tobes was king of then!), but it did slip and I often had to get off and readjust it.
I have to say, having a treed saddle (fitted by our own wonderful Poohsmate), it is SUCH a relief that it doesn't move at all. I often have to get off to do gates and get on off the ground, we can gallop, go down some of the steepest hills I have every come across (and up), and that saddle doesn't budge.
But if it is treeless you are set on, I would just make sure that the pressure distribution is OK.