Biggest Bitcoin problems in my opinion:
In my opinion, to have a full bitcoin economy, one should have easy ways to convert fiat currencies to bitcoin so they can easily buy it at the current market price.
As for the 'problems' you mention:
- Too complicated for masses
The same was true for email in the very beginning.
- No 100% reliable back-up protection if private keys are lost or stolen
Custodial services are available, but I personally do not recommend them.
- No charge-back option (this is both pro and con)
This is a strength, not a downside.
- Not enough big merchants
I agree with this.