Not dead, I'm here! :-D
From my little experience, different platforms suffers different drawbacks. And all the Qt performance tricks are applicable with QML (Qt::WA_OpaquePaintEvent flag for example).
What I found to be performance killer on all platforms are graphical effects and transparencies. It makes sense as there are some computations and commpositions to display each object and low perf platform cannot cope with this.
In conclusion, Qt and even more QML as any other high level tools, hides loads of operation done in the background and sometime some small modifications in what you ask (QML animations for example) can lead to huge impacts in performances and depends on the capacities of the platform, this is very difficult to analyse...
NM