The environment being devastated is mostly a fair criticism. China is making huge efforts toward sustainability today, but maybe too late to make enough of a difference.
Hundreds of millions died, yes, that's what happens when 80 years go by in a country of hundreds of millions. I don't see your point.
How do I feel about their treatment of indigenous Tibetans? I think China did a heroic thing by ending slave labor in Tibet. I'm not sure if they took the best path toward liberating Tibetan slaves while also not erasing their culture. I don't know if such a thing was possible, and my personal opinion is that if it wasn't, it's preferable for them to be free and not keep a reactionary culture than keep their slave state intact, the same way I don't care about preserving Confederate monuments in the US despite their cultural importance.
All in all, for a real, existing country, China has a really good batting average.