This is a loaded question.
Are they safe for who??
If we are talking about a couch potatoe then no! one work Rhabdomyolosis.
If you are in decent shape with no musculoskeletal ailments or compensations then sure it will be fine…as long as the trainer is decent.
The bottom line is, if you personally can handle it and the trainer is good then go for it. If you are injury prone and the trainer is mediocre at best then avoid it like the plague.
As far as the anterior chain post, I don’t know abou that, then do a lot of deadlifts, cleans, snatches, squats, the rower, and pullups so I think the posterior chain is covered. Again it all depends on how good the trainer is or isn’t.