What kind of flooring is more of a preference, each human has their own, horses are fine either way, as long as managed properly for whatever kind of flooring we humans choose for them.
After over half a century with horses in all kinds of flooring, concrete is my preference any day and, for what all those years, centuries really have shown us, so do most professional trainers, that can tell horses are fine with the right management.
Horse’s legs and feet are made to stand in place and flat hard ground is ideal for that.
When it comes to lay down, they prefer a soft place.
With opposite needs for standing and laying down, horses can choose where to stand, where to lay down when turned out.
Watch your horses and you will see, if there is a concrete pad they can access, like some pens by the water trough, they will choose to stand there, not on the nearby to us humans softer dirt.
When laying down, they will go to that soft dirt for their recumbent naps.
We have concrete stalls and sand piles and horses never stand asleep around the soft ground of the sand pile, unless others are napping flat out on it, but choose to go where the ground is hard and flat to nap.
Uneven and soft ground is a choice for some sore footed horses.
If they stand in a soft spot, may ought to check to see why.
Once at a friend’s place, I noticed one horse was standing on an uneven spot in their pen, looked a bit off.
Sure enough, horse was getting sore, navicular was becoming a problem, vet diagnosed and treated him for it.
Watch your horses, some may indeed prefer soft ground and be ok, but most here have always preferred hard ground to stand on, even bare concrete fine with them for that.
One good compromise is, have concrete and matted and well bedded stalls, where horses can have places where it is harder and still a soft place to lay down for naps.
Let your horses tell you what they prefer.
One human advantage of concrete is, no maintenance, don’t have to keep straighten and leveling the floor, ever.
If some believes concrete floor is bad for horses, then of course for them that leaves out concrete as an alternative.
For the rest, if not sure what you want, either way will be fine.