I’ve had a tack locker in the stall. Didn’t think I’d like it. Learned to live with it. Would not choose it on purpose, if I were building a barn. It probably wouldn’t be a deal-breaker for me as a boarder if executed well.
Some of the issues.
These lockers only opened into the stall, which meant loading, unloading, and cleaning tack was suddenly a much more iffy chore. Here’s the scenario. You have a sliding stall door and you want to get your saddle out of the stall, while your horse is in it. This requires opening and closing the stall door twice, once with a saddle, and you have open and close and get through the door with the saddle in your arms. You also have to be able to open and close the locker with a saddle in your arms. Do the experiment: it feels risky for the saddle and risky for the horse getting out if you have a horse that is anything but super mellow and a human that is less than strong and coordinated.
Cleaning tack right on the hooks is easy in a tack room, not so much in a stall with a curious horse and no water at hand. Thus the tack does not get cleaned. Since it’s hard to take in and out of the stall, it doesn’t go home or go out to the aisle to get cleaned either.
Cleaning feet and legs, and putting on any kind of wrap is doable in a bedded stall, but so much nicer in a clean, matted crosstie.
If the locker can be completely accessed from outside the stall, the issues are better, more in line with how an exterior locker arrangement works. Without moving parts inside, you maybe have fewer safety issues. You could also maybe have a small door inside for getting access to a brush box, and expect that the exterior door would be used for accessing saddles and bridles.
I’ve also worked in communal tack rooms and situations where each stall had a separate locker not attached to the stall. All the situations have downsides of security or not having enough space of the right shape. Probably if I was building a boarding barn I’d make individual exterior lockers and have them on some sort of electronic locking system that made it easy for grooms and trainers to access any horse’s equipment, and for owners to get their own equipment, but kept other people out.