I'm have what's kinda in-between self-care and full-care.
I have my horse about 10mins away from me at a barn with 7 other horses/ponies. The couple I board with feeds and turns him out during the day. At some point during the day or early evening, I clean all 8 stalls (they provide shavings) and if a water bucket is dirty and something else needs or should be done, I'll take care of it. So a couple weeks ago when their gelding dug a hole to china in his run, I found it, so I filled it in. When my horse escaped and ended up with a minor gash, they cleaned and treated it, then chained his gate.
They blanket/fly mask/whatever as needed but if I am there and the sun comes out and it's hot, I'll pull off everyone's blankies/sheets. Or if the temperature suddenly drops, it's starts hailing/pouring/whatever, I'll bring everyone in or throw blankets on. Every time any of us are walk past a water trough, we check and clean/refill if needed. If the horses are stuck inside for the day, I throw everyone extra hay while I clean stalls.
They provide grass hay and beet pulp, breakfast and lunch are served in turnout, dinner in stalls. My horse needs alfalfa, I store it at my place, haul in 10 bales at a time and stack them in the corner of the barn. Since breakfast/lunch are done in turn-out, I had to accept he would share his alfalfa with his turnout buddies. He attacked one of his buddies, so dodohead ended up with individual turnout but he still manages to share his alfalfa through the fence with an ancient pony. It is what it is.... I can't expect them to stand out there and tell him to quit sharing!!
Since my horse gets almost 2x the quantity of grass hay as everyone else, I haul timothy hay over (I have horse-quality hay for my cows) and he gets half timothy, half their grass hay (their grass hay supply is limited due to the hay shortage here). I provide his grain/supplements.
Whenever I go on vacation, they do everything. When they go on vacation, I do everything. Since most of the horses have very specific dietary needs (IE soaked for the toothless, grain 3x per day for a thin rescue, alfalfa and 2 kinds of hay for my guy) we both prefer it this way as there are zero worries about feed or horse mix-ups. The BO hired a "professional vacation care provider" last year during a trip and he mixed up a white Welsh with a bay Morgan despite having been left a guide with PICTURES! Thankfully they were eating basically the same diet.
It works out well for us but then I am the only boarder. I could see issues if there was multiple boarders. Nothing goes missing or gets taken.... other than occasionally if someone escapes I'll find bites taken out of my alfalfa bales!