Opera should separate all these stuff, or at least give the option to download the barebones browser without all the extra apps embedded. It’s a pity, because from the technical point of view I find it to be one of the most advanced browsing engines, if not the most, along with google chrome.

Opera could have spent the development effort in polishing the UI instead and making it feel more modern and nimble.

In response to some of your questions, so many many people complain about Opera being “bloated”, which I’d understand if any of it’s extra packaged features impinged on efficiency. They don’t. There’s a good post on Unite’s performance here:
http://unitehowto.com/Performance

The above page also has info on security, it essentially uses a sandboxed virtual filesystem. Your point about it being dependent on service developers is still valid though – people could develop malicious Unite services, but even these would still be 100s of times more secure than an equivalent malicious Firefox extension could potentially be.

Privacy wise, it’s essentially mostly public that I can tell as it’s http, no SSL, which concerns me to be honest. But that’s the same with any http site on the web, beyond that, not everything necessarily goes through Opera’s proxy. This depends on the service. Of the initial 7, 6 of them go through the proxy, the media player is p2p (apparently).