Two things I'd leave to dedicated shops vs the dealer - wheel alignment and glass. Safelite are plenty good, in my experience. They know what they are doing, and have all the needed equipment. I don't think a BMW (or any other brand) dealer is better at replacing a windshield than Safelite or any decent body shop is. Many dealers actually outsorce this.
The problem is not Safelite, but the windshield that the insurance company makes them work with.
Insurers will always proceed with and push for 3rd party glass (which could be good or not so good), and will only go to name brand (BMW original in this case) if you agree to pay extra, OR if the 3rd party does not cary that specific model windshield.
The good part is that most exotic windshields will not be carried by the 3rd party and the insurer will be forced to go for original. And if it's at least half-good - they'll treat it as their own 3rd party. This happened to me with my econosuv - turned out that my windshield was exotic enough. They covered it in full.
Initially, when they thought that they had a 3rd party for my car, they were quoting me $250 (or was it $300) extra if I wanted to go with an original from the dealer (which is still less than the actual difference in price I think, looked like a flat fee).
