Had no real issues using my card in Rio. Most places we went accepted cards, and only at a local pharmacy was my card not accepted. Quite a bit of cash usage as well, especially taxis and beach vendors.
I didn’t personally take out any cash, but my partner did (using his Starling card). Our whole traveling party seemed to have issues with finding ATMs that worked. Santander ATMs definitely worked for everyone. Several of the local ATMs did not work for any of us (trying a mix of VISA and MasterCard debit and pre-paid cards from the UK, USA, and Australia).
A friend of mine believes her card was skimmed at an ATM (she saw transactions she didn’t recognise a few days later). I guess in general one should always be careful to check that machines aren’t tampered with, but i wasn’t there so I have no idea whether it was possible to see evidence of this. In general ATMs were heavily guarded within bank buildings rather than outside like we see more of in the UK.