Well… from what I know of things, usually it’s not a problem, BUT—
If someone in the company has a major, catastrophic-level claim it could impact all of you - that would be, say, an employee or covered family member with cancer, or needing an organ transplant, or a major burn or equivalent trauma, or someone has a kid born with a really bad birth defect. In the past, this has resulted in rising (one might say skyrocketing) premiums for everyone in the insurance pool and/or the self-insurance collapsing. I’m not sure what safety nets, if any, are present for self-insured companies these days. It’s not a common problem, but it does happen.
Another to watch for is perverse incentives. As an example, one time I was covered under a self-insured employer the policy would only cover pregnancies with complications. Regular, uneventful labor and delivery was not covered, not one penny. Suddenly, all the women having babies under that policy were having “complications” and winding up with cesareans, which were covered in full. Naw… couldn’t possibly be a connection, must be coincidence, right?
So yeah, there can be pitfalls. If the company is self-insured then everything covered or not covered really is the employer’s choice, and since the employer is not an expert in determining what’s really most likely or cost-effective over time, well, it’s not always ideal. It might be, especially if the company did some homework. The insurance company devoted full time employees to analysing risks regarding health care coverage, it’s unlikely your company employs actuaries for this purpose.
So… hard to say. Like I said, it’s usually OK. In general, the bigger the company the better this will work out (because the larger the pool of covered lives the more the risk is spread out). If General Motors self-insures I wouldn’t worry too much. If a 100 person company does, huh, I’m not so comfortable with that.