Years ago, when the real Buck was riding sound, he had a back like a sofa and was built for bareback riding. No withers, double back (his back was so muscular his spine was recessed), and gaits as smooth as silk.
I rode exclusively bareback for over 3 years - 4-6 days a week, mountain riding, and galloping and jumping small obstacles/creeks/ditches every week. The only time I used a saddle is if we were packing into the mountains for a full day or overnight, or if we were doing gymkhana or team penning. Showing low level dressage or low fences/fun classes I pulled my stirrups off my saddles.
I was in fantasic riding shape. I could post easily and maintain a 2 point position for about 3 laps around the indoor bareback. My lower leg was remarkable and my core became so strong and stable it was like I had a plumb bob for a spine.
I did however develop a few very bad habits, and most took years to realize:
• I forgot how to use stirrups. More accurately, my leg joints and muscles forgot how to act like shock absorbers. When I went back to using a saddle, it took almost two years for me to put stirrups back on. As I started riding other horses who moved and responded way different than mine, it became a bigger issue and my legs had to relearn. I also resented (and still do to this day) shortening my stirrups for jumping, I feel out of control and wobbly with a short leg. I’ve forgotten how to keep my center of gravity low with a short leg.
• I forgot how to sit evenly on a circle 20m or smaller.
• My body acclimated to my horse’s subtle unevenness, so forever after, being even feels strange, saddles that fit well and sit squarely feel odd and twisting to me.
• When the horse stumbles or throws a wonky move I tend to grasp with my legs and hands.
• I tend to roll my shoulders forward when things get fast, and my hands stay fixed when jumping - I never developed an auto release, and instead used to bridge my reins for balance when things got wild on the trails and/or over obstacles. Its a habit I can’t kick now. I also tended to want to ride with my hands in my lap, though years of carriage driving now has given me the opposite problem.
• Despite not being a big mover and extremely smooth ride, being truly through presented a much springier ride and active back muscles than my seat could manage bareback. When he was ever so slightly hollow, it made for a nice nest for my seat. Engaged over his back was like trying to ride two porpoises. So I encouraged the easier ride, allowing my horse to take the easy way out and fake his collection and engagement.
• I ruined every pair of jeans/breeches I owned through permanent dirt/sweat stains.
I would never take those years back though, I had so much fun.
My advice would be to make a note of what feels strange when you go back to using a saddle, and examine why.