Misalignment would be a problem. In that case, I think I'd be tempted to send the rifle back to the factory. Didn't see it on the one M77 I own, but it's a very early rifle and maybe I was just lucky. Gotta figure some lemons come off of every production line.
Fragile -- having handled these in person, I'd rate them as very sturdy. Not pretty, but typical semi-chunky Ruger and quite sturdy. As 256M-S says, not what you'd put on a Barrett .50 but he's right IMO that there is a culture that's developed that really over-buys rings/mounts. Having indestructible mounts and rings doesn't prove much if the optic itself is impaired or destroyed by a mishap, unless maybe you want to make sure you can remove the optic and use the rings like a pair of ghost rings. I have rifles set up with the Leupold DD rings that get laughed at in some quarters as fragile, but I don't see it. There are places where you need the F350, but going into town to get groceries ain't one of 'em.