It's reliability as well. The speeds these things normally run at plus the fact that a typical upgrade trajectory will overspeed the compressor implies these compressor wheels will be quickly pushed to their design limits.
I have confidence that the Garret turbos have been designed and tested with necessary margin to accommodate at least normal operation for the life of the turbo. I seriously doubt these new billet wheels have been learned out to that level of performance or reliability. Through luck and perhaps some judgement this new design may be just fine, but I wouldn't recommend it based on the minimal information available right now and my limited understanding of such details. Worst case the compressor performance is subpar and the billet wheel blows up like a cheap flywheel with fragments getting sucked through the intake. Such consequences need to be taken into consideration when making the decision to upgrade to these billet turbos.
