1) social security taxes
The government has carved out a special status for these Phd "employees", which means that social security taxes are not paid for these employees, and of course the resulting burden is carried entirely by the employee. Ironically, and because this results in a higher effective wage for these young people, who are too young to care about pension and generally don't need health insurance, they actually like this part.
a) you can't get unemployment if you get fired, or after you "graduate" (which amounts to firing with a nicer pink slip for 9/10 Phd employees)
b) you don't build up pension rights
c) health insurance is the employee's problem (though many universities "solve" this through private means, but it means significantly less coverage)
c) No paid holidays (again, universities sometimes make up for this, but not the same as private sector. This is pretty much the only European job I know that doesn't come with 4 weeks paid holidays. This is legal, because it's not a "job", legally speaking)
d) very few other employee rights (e.g. no rights to unions, even though I doubt any union would stand a chance their anyway). No rights if you get fired. If a university downscales (thankfully, doesn't happen often), these people are treated worse than contractors.
2) They are most definitely not "civil servants"
The legal differences and problems this poses are too numerous to go into. Let's just say, once again, the Phd student is the loser here.
3) about the one thing these people do have going for them is that government sponsorships are tied to the person, and include a (small) amount to the university and the pay for the Phd student him/herself. So if the university fires a Phd student, they lose the funding.
In other words, unless you commit a crime or something as a Phd student, if you're on a government scholarship, getting fired is not a concern.
Being a Phd student on either a university or private scholarship ... you don't want that. Or, at least, make DAMN sure you like your promotor.