Rio wears them for fast hacks and jumping, I find they don't make his legs as warm as brushing blots. As for welfare issues I wouldn't say there were any if put on correctly with the right tension/not over tightened.
Our horses always wear boots when being ridden. Hacking out they have brushing boots & knee boots. Show jumping they always have tendon boots & fetlock boots & over reach boots. They are very athletic & an over reach injury can be dangerous. I don't see any issues wearing tendon boots, most are open fronted so tendons don't get too warm.
mine always wear boots all round. She wears brushing boots for flat work, and open fronted tendon and fetlock boots for everything else I would much rather spend £40 getting a new set of boots, than spending god knows how much on vets bills, and putting my horse through all that pain, box rest etc. I think its a no brainer
I struggle to find tendon boots that fit my horse well the seem to bulge over his joint and sort of stick out. I've tried a few kinds, with Velcro and buckles with elastic, am I just buying the wrong ones? My horse brushes but would quite like the extra protection for fast hacks and for the day I actually decide to leave the ground and jump afain
I get weird looks when I boot or bandage my two before I ride. Neither of them brush or have even come close to hitting themselves, but as PingPongPony said I would rather spend a little money on new boots than a heap on a vet bill that could have been avoided.