Ex civi (DWP) who was outsourced to a private company and didn't really notice a lot of difference!
Just as in all walks of life, it isn't black and white. The civil service is viewed as inefficient mostly because it is large but moving over to a large private company, I wouldn't say I noticed a huge leap in productivity.
The tangible benefits like flexi, some extra holidays, a half-decent pension (well, it used to be!)and relative job security have mostly been matched by the larger private sector employers but usually better salary.
I'd hazard a guess that most of the problems with the Civil Service is caused by their political masters who change policies at a whim (and this applies to all political persuasions).
Now, if you're comparing the civil service to the tiny [private] company I work for now, there's no comparison…and no benefits 🙁