An important concept to grasp with Share is that it uses exactly the same mechanisms as the Explorer client - i.e. it's exactly the same Repository. Share doesn't yet have all the functionality of Explorer, as (a) it needs to be exposed via REST APIs and (b) it's a new user interface to design for.
In terms of your questions:
- Improved workflow support will be coming - probably not in v3.3, but in v3.4 (if there is one, or v4.0). This is due to the Workflow API needing to be refactored to use the Forms Service.
- Share doesn't currently support custom workflow directly. The My Tasks dashlet will attempt to render tasks for the current user with transition buttons, but may not work in all cases. The Document Library only directly supports the two out-of-the-box, single-user workflows "Adhoc" and "Review and Approve".
- Explorer-based workflows are likely to work without issue, but "your mileage may vary" until Share supports them natively.
Thanks,
Mike