I've found good and bad:
Good - I like having elevation profiles and accumulated elevation.
Bad - Doesn't always distinguish between paved/unpaved roads and occasionally will go out of its way to ride a short section of trail.
And to show you how hard route mapping can be for a machine serving lots of people? Your "bads" are "don't care" (gravel v road) or "good" (cycle paths) to me. I think cycle routing is MUCH harder than routing for cars, where people generally only care about shortest distance or shortest time. Personal preferences are much different in a cycle route than easily computable facts.
As to Google bike routes, I find it very hit or miss. Always best to double check its math instead of just blindly trusting it. Just another tool in the box.