I bought about 40 thin cows last summer. I am down to just one cow and calf left.
Downside--- had lots of losses. One just disappeared at turnout. Cows that came up lame- never gained a lb.  Couple died. Had one(the nicest one) get hung up in a fence and choked herself to death. Pinkeye broke out(hadn't had to deal with that in YEARS).
Upside--- 20 of them had calves unassisted. I had planned on just supplementing them on fescue. It wasn't enough to put the weight on and have a calf. So I ended up feeding them a TMR.
Still ended up making more off them than the set of calves I bought at the same time. 

going to make some changes  and give it another go this summer.

• more aggressive pinkeye protection
• on TMR the whole time
• if they develop a limp they leave for market the next day no matter what