I hear that supposedly some details of the coverage kind of encourages some unethical treatments.
The same thing can easily be said of dentists in the U.S. Apparently, it's particularly bad in states like Florida, where dentists can refuse to treat you at all if you don't agree to every treatment they recommend. People often get pushed into unnecessarily expensive-- or just plain unnecessary-- treatments.
As far as U.K. dentistry goes, my husband and I have had great dental care here, both in terms of routine check-ups/cleanings and for bigger issues. My husband has had two root canals, and I've had two broken/chipped molars filled & repaired, and a bridge repaired. Never once has anyone suggested just pulling anything out.
As vadio said, ultimately, you're in control of your own treatment. If you'd rather they do something differently, say so. If they can't or won't, find a different dentist. Just be aware that, if something isn't offered under the NHS (usually stuff considered 'cosmetic'), you might have to pay more for it-- but even then, it'll be a fraction of what you'd pay in the U.S.
If you're offered dental coverage by your employer, take it, because why not? But it's not something I'd spend extra money on, unless you've had extensive dental issues and would be looking at things like implants or cosmetic dentistry. And in that case, make sure those things would actually be covered under the policy before you sign up.