The older ones that only worked by slamming the door closed are almost worse than useless - they're commonly disconnected and they can wreck the door over time, if not properly adjusted.
Two issues:
- Do they close the door, unassisted, from any angle of opening, overcoming the resistance of the latch (if fitted) or any seals?
- might they compromise the fire resistance of the door?
The first is easily tested (assuming you can open the door), and for an existing installation (where as a fire risk assessor I'd propose that it's not for you to worry about CE marking etc) if it functions correctly then it should do its job in a fire.
The second is less straightforward - ideally you might be able to check on the manufacturer's website to see what they say about compromising the fire resistance of the doorset. Some do have "Certifire" certificates that tell you all you need to know about this aspect of performance - some don't!
Of course, checking this stuff ought to be part of a planned maintenance regime (like any other item of fire protection) - BS 8214: 2016 give guidance on what this ought to include, plus recommendations for repair. It's not particularly challenging to achieve, if done by competent persons.