It depends on the employer. If you're under 35, and in decent health, you're likely better off getting disability and life insurance on your own. Your premiums will be lower than from a group plan, which usually takes some average demographics to determine the group premiums.
If your employer is paying the premiums, you're better off if you can decline the benefit, get yourself a lower premium plan, and pocket the difference.
Conversely, if you're older, or have health issues, those group plans can be a steal of a deal.
That may be true although, to the broader point, I've never seen those sorts of benefits be optional (albeit across a fairly small sample size). Employers get a good deal precisely because there's going to be a lot of adverse selection with these sorts of things. Healthy single 20-somethings get a pretty good deal on their own but most of them won't even bother.
Conversely, if you're older, or have health issues, those group plans can be a steal of a deal.