Having worked for a CMG, the happiest folks tend to be: 1) the new grads who think the job is awesome as they have nothing to compare it to and 2) the physicians admins. Once you work for the CMG for awhile you realize there's no real path for things to get better for you over time, other than trimming hours or trying to become an admin yourself if you can stomach it. Along the way, the CMG will slowly demand more from you and your time without giving you extra resources or compensation--rather, they'll try to cut these things. Every year will be like groundhog day in this regard. It will become painfully clear you're essentially a cash machine for non-clinical people who really couldn't care less about you or your patients.
That said, the daily life of working for a CMG really boils down to the site you work at. There are EDs in the Team, USACS, and Envision portfolios that aren't terrible to work at. Then there are sites that are raging dumpster fires with a seemingly endless supply of fuel.
In my opinion, if you have to work for a CMG, the best way to protect yourself is to work at one of their sub 20k visit sites. Generally, they have decent staffing and the metrics are a bit less of an issue. Or work at one of the major tertiary sites the CMG highly values -- at these places the CMG knows if they pull major shenanigans they'll lose the contract. Working at these two ends of the CMG extreme will make your life easier on many levels.
The worst place to work for a CMG are their 40-60k sites with limited specialty support and where the ED is outsized for the hospital and you have to transfer frequently. You'll hear about metrics but have no resources to satisfy them. These are the places where the CMGs will most readily cut MD staffing and replace them with midlevel hours. Your liability and work load will increase and your pay will not. The CMGs will do other things without regard to you or your patients' well-being if it improves their short-term bottom line since they don't really care as much about these sites.