I'm a 911 dispatcher, I've talked people through CPR countless times over the phone, I have very little confidence that most of them were doing it properly because CPR really is pretty brutal, I've taken a lot of CPR classes over the years, and every instructor I've ever had has mentioned that if you're doing it right there's a very good chance you're breaking ribs in the process. Unless you've actually had training and have an idea how rough it can be I doubt that most people are going to do it hard enough out of fear of hurting the patient.
I've luckily never had to do CPR in person myself, although I was once on-scene while it was being performed. I was at a party, someone came inside said they think someone died out front, I went out to see what was going on, came around the corner of the driveway and my friend was already doing CPR on a guy laying in the street who crashed his motorcycle. I know my friend also had CPR training so I let him keep at it, I stood by to relieve him in case he got tired and started counting to make sure he was keeping a good rhythm. I of course know my share of cops, firefighters, EMTs, etc. who have had to do CPR in their line of work, but I don't exactly press them for any details about it, but I talked to my friend afterwards to make sure he was OK, and he talked about how he could really feel the guys ribs popping as he was doing it.
It was also a pretty good illustration of the bystander effect, when my friend got outside there was already one or two other people pulled over with the accident but not really doing anything, not checking on the guy, not on the phone with 911, just kind of standing there. If you asked them, I'm sure they probably would have said they were blocking traffic with their vehicles or something, but that doesn't really do any good when the guy needs CPR immediately.