Android Skype seems to work quite good (half a second latency) but the sound output is always on speakers. Also the proximity sensor does not work with Skype.

While N900 has working jabber videocall, it is not encrypted and does not work through most firewalls.

Jingle relay node on the XMPP server is required for audio to work properly between internal and external networks, and I assume that every internal network would have to have one of these to support the scenario where anyone could call anyone else, from everywhere.
Unfortunately for example OpenFire's jingle plugin did not work at all when I tested it month ago. (OpenFire is used on the server I'm hosting.)

XMPP audio and video generally is a pain in the bum, as it uses streaming port auto negotiation on specific range of ports, and uses only UDP for the streams. Most of the companies block both inbound and outbound traffic, cutting out all XMPP streams in the process.

Skype goes around this problem by using any open ports it can find. In addition to this I suppose it uses, from the streaming view point, more technically inefficient TCP or TCP wrapper for streaming. Inefficiency is neglible when the network speeds are good these days. Again I suppose Google Talk uses the same method for their video call streaming.

Common XMPP audio+video is still pretty much work in progress IMHO. For instance Windows client of Pidgin does not support it yet, and on computers only Jitsi client supports encrypted videocalls (and is a bit flaky especially in its stable 2.2 line). Of Android clients nightly build of Jitsi seems to be the only XMPP client supporting video calls if you exclude Cisco's proprietary solutions for their own gear.

After saying all this I'm still going to test android version of nightly build of Jitsi (see jitzi.org for details) behind the firewall next Monday. It will be interesting, but I really don't expect it to work, as I can't recall it working on "real" android either.

Of course I'm hoping that Jolla will get Skype and Google Talk video calls working too, but especially with Google I expect them to slowly move to their evil native and proprietary Hangouts implementation. And we are not likely to see that on Jolla (nor on any android fork).