One of my students has her own saddle. It's a very cheap Asian POS. I let her ride in it with a thick Skito pad. She's only about 65 lbs and rides for about 45 minutes once a week. It "kind of" fits the horses she rides. When she gets in to more demanding riding (she's only walk/trot with a tad bit of canter right now), I'll make her use my schooling saddles (all good quality and all fit the horses they're used on).
I used to have my own saddle before I had horses, but I rode horses that were all about the same shape. I checked the fit before I rode, used different pads if needed, or used another saddle if mine didn't fit. You can get a saddle that will fit many horses, but definitely not one to fit "all". Even treeless saddles need to fit a horse, and those changable gullet saddles have to have the right shape and panel type to fit.