The one we use is on rubber, which is swept every other day. The rubber matting is replaced when it starts to wear down.
I would say for the yard I help on its invaluable. The horses go on twice a day, once each way, for the same length of time. So therefore there is no worry about them becoming one sided.
We had a horse on box rest. Big strong 18hh. He was good to hand walk, but very strong, so when the vet allowed it, he started building it up on the walker. This made him less fresh, and hes now back to being ridden and is being a complete gent. However I believe the routine of the walker helped him.
Its also been great for a little mare that we can't ride (rescue mare) shes just a companion, but is a very good do-er. Because she comes in every day, she can go on the walker and be lunged/long reined when time allows. This has helped keep the weight off her, and we dont have to worry about lami.
It also saves bother for if we've bathed the horses, or theyve got hot when worked, to cool them down. Especially as a lot of horses are ridden per daily, it saves the YO time.
Completley invaluable peice of equipment IMOH