It is not clear that any of these are or will be problems.

Deflationary
Its a feature, not a problem

Slow transaction times
Perhaps too slow for risk-free face-to-face transactions, but that is not necessarily a problem, and it can probably be solved if it is.

Environmental impacts of mining
The amount of energy spent on mining will generally always be less than the block reward plus transaction fees. The block reward halves every 4, and so will the energy consumed.

1Mb fixed block size
I think it should be increased now before it is too late. Others do not. Either way, it is not a problem.