I think it all really depends on the parties involved.
For instance, I got my mare because an older friend/riding instructor/mentor person knew of Lacey's situation, thought she'd be a good match for me, and felt like I was ready for a horse.
I met Lacey, we "clicked" instantly, and my friend offered to own+pay for Lacey's needs for a year, then transfer ownership to me at the end of the year if I had consistently shown that I was committed to Lacey [by working with her 2-3 times a week, etc].
Lacey, however, was a free horse for my friend to "purchase." My friend actually even got a few tons of grass hay, a saddle, Lacey, and Lacey's BFF horse [who my friend saw potential in as well], just for taking Lacey.
Anyway, that arrangement worked out really pretty well for Lacey, my friend, and I. In a few weeks it'll be my 5 year "anniversary" with Lacey, from that first day I saw her.
Basically I'm super glad I had the chance to get Lacey and I wouldn't give that up for anything, BUT the parts involving my friend did get slightly sketchy towards the end of that first year.
Lacey's a hard keeper in the winter and needs alfalfa to keep weight on, but my friend was not willing to do that [she believes that alfalfa "kills horses by turning their stomachs black"] and, at the time, I didn't know enough to just go buy my own alfalfa.
She also refused to get Lacey any sort of hoof care in the winter, for similarly strange reasons - "horses need more hoof in the winter. Hoof=traction and in Oregon mud, we NEED traction!" I ended up doing a little hoof care myself but I seriously had no idea what I was doing. No one should have let me near poor Lacey with a hoof rasp!!
My friend also let a variety of people ride Lacey who should not have been riding a green, anxiety-ridden, hot horse. Most of those people ended up falling off due to their ignorance which I can't say helped Lacey's training. But what was I going to say? "I really wish you would stop letting people ride your horse that you pay everything for, on your property, because I'm going to own her in year"?? Hah.
Just that sort of stuff - when someone other than you owns "your" horse, they really have you in a corner if opinions disagree on care.
It was all worth it for me, in the end, but it was a hard year. I've now moved Lacey 'home' -to my neighbor's house- and I'm able to care for her "the proper way" but if we had stayed at my friends house, I can guarantee that that person would still be hanging on to Lacey like she owned her still.
I guess it's like what "they" say about having a romantic relationship with a friend - if you want the friendship to last forever and the end of that friendship would really kill you, don't get involved like that. But if you don't mind if the friendship ends, do it.
That friend and I are still "friends" but we're not friend-friends like we were. I see her maybe once a year and it's always awkward. We never had a falling out of any kind, just our friendship couldn't sustain the differing opinions and just different people-ness.
And boy, this ended up waaay longer than I intended.