I took a truckload of cats to a low-cost place run by a dog sanctuary a few years ago (they're closed on Wednesdays so their full-time vet can do spays/neuters for the public). Of course a vet at a sanctuary is going to be pretty good, so I don't know if my experience is typical. But he did a great job. . .small, tidy incisions, dissolvable sutures, antibiotic shot, etc. Better than any regular vet I've ever used.
There's a local vet who recently started doing low-cost alters for farm and feral cats ("farm cats and ferals for fifty!" is his tagline

. But if you have more than 10 to do he does it for even less). I haven't used his services yet but I know someone who had their 20+ farm cats done, and they didn't lose any, at least.
I've never had a dog done by a low-cost place. But every dog in the family was done by a different vet. . .oddly.
Anyway, bottom line is, I'm sure the quality of a low-cost is just the same as the quality of any vet--some are good and some aren't so good. But probably those who volunteer their services like that are on the higher end of that spectrum.