That seems like a bizarre reason not to use it, given that it's strictly a performance optimisation, and browsers will simply ignore CSS rules they don't understand.
I imagine that it could cause accusations from Apple that Google is trying to take their customers out of the ecosystem, putting a nice cash flow at risk.
It's a bad argument because if someone else makes a better product then they deserve to get more users. Nothing's stopping Apple from implementing those optimizations into Safari. That's the whole reason we have competition. If the government is going to make it so that improving your product makes you liable to lawsuits then no one would ever improve anything.
It's not the government dictating the lawsuits, I was referencing the deal between Apple and Google to have Google as the default search engine on Safari.
It causes a new stacking context which has significant impact on how everything is rendered and things like Z-index behave. This can cause head-scratching issues and differences which aren't obvious at first sight.
