A treeless saddle only slips if not properly tighten.
Last august while riding my arab Gaaylah (11 yr old 14.1 SER arab mare) in my sports saddle (treeless) with breast collar and my friend was riding Zar (14.3 hh arab gelding) in a wintec endurance saddle (yes that has a tree) we were charged by a deer. Now, both of us came off. I am not sure how I did as I don't remember much. But my horses saddle didnt slip. My friends saddle did. And I out weight her by oh.. 60 or 70lbs. (I was 240 and she was/is 170? Maybe less). And my comign off had nothing to do with my saddle (more like I was knocked in the head because gaaylah bucks a liek a bronc coming out of the chute.
I have ridden in my sports saddle for over ten yrs. I love it. My horses love it. I endurance ride in it. I trail ride for multiple hours in lots of different terrain in it. It is light (about 25lbs with all accersories) and very very sturdy. I have had it on a multitude of different backs, withers and shoulders. The only one it had trouble fitting was my mom's "built like a brick ****house mutton withered (ie non-exsistent) arab/app gelding. I have trouble getting a full qt bared treed to sit right on him. Its like sitting on a really really wide slick barrel.
I feels different to different people. Of course I always used to ride bareback so maybe it don't notice how different it is form a treed saddle. My mother didnt care for it, hated feeling the horse's muscles move. To me that gives me an extra microsecond to counter act any spook/buck/bolt/naughty behavior that they are thinking out committing.
Price ranges- very cheap ($100) to very very expensive ($3-4000). I got mine off tack trader.com, second hand (and I am still using it 10 yrs later this is a very sturdy saddle!), second user for $500. Sports saddles hold their worth (new they run from $900 to $1200 depending on type and extras).
The horse in my icon is wearing my saddle. That's Magic, my 10 yr old space alien monkey magic carpet ride. I'll post a pic here.
I am using a cashel softsaddle- saddle pad with it. And now I have my thinline to put under it to help with cooling. Magic is a real sweater.
I highly recommend doinga google search. YOu can get postings to other threads on different websites about treeless.
BTW mine isnt a "true" treeless. Its a hybrid. Since now they have they ahve the "true" treeless (like the Senasation and the Ansur) mine is considered a hybrid. Of course those "true" ones didnt exsit back whenmine was on the market brand new. Course they still make them this way. Mine has a solid cantle and pommel. Still it has no "tree." distinctions distinctions!