Back when I first started mixing ITB around 2000 I got all excited on a project when I realized we could just bring the PT rig to mastering. So we did, and I was ready to make mix adjustments if the mastering "ate the snare" or pumped the bass too much, etc. Turns out we didn't do anything but a direct transfer minus my temp mastering plugins.
I guess I would describe that project as something beyond stem mastering, but about as unnecessary IMO. Thing is, if your mix rocks you really don't need much (if any) mastering. And while I understand that is an extreme example, it just means the worse the mix the more likely something like stem mastering will help.
BUT, there is no free lunch. Turning up a vocal stem means there is more dry vocal and less reverb. Unless you also have stems for the reverbs from each track, which means an additional stereo stem for each track in your mix that uses any FX such as reverb or delay. There is also the issue that if the mix needs that much help, stems may not be 'granular' enough to fix all the issues. If your kick is too weak in the drum bus, you need individual tracks to address the issue.
But at that point, you ARE mixing because you are adjusting balances, which again points to the mix not being finished which begs the question "why are you mastering an unfinished mix".
And yes, there are exceptions to all of this, and I'm sure having stems at mastering has saved a project. But seriously, with the ridiculous simplicity of recalling a mix and making a quick change, the BEST solution is to fix the mix - and its super easy to do, so if your mastering engineer says they want to do something that is going to mess with your mix, better to adjust at the mix than with stems IMO.
All to say, in 20+ years of having near instant recall at my fingertips, I've found it much easier (and better in the long run) to adjust the mix in the rare cases there was any problem at all in mastering.
My suggestion if you're not sure about your mixes is to send them to your mastering engineer for feedback, try making any suggested changes, and send both version to mastering if you're still not sure.