Pretty sure the mechanic did. I've ran it for about 5,000km without issue.
After typing that out, I realise I probably should change the cassette/chain, though in my defence I have looked after it pretty carefully. Still shifts pretty well
Still, would like to know why the cassette is wobbling.
is the lockring tighten up enough? can you find out if there's a spacer? with the Ultegra hubs, they need a small thin spacer as well as the spacer to run it on the 11 speed freehubs.
Did you fit a spacer for the 10 speed cassette to take up the space that normally take the 11 speed cassette?