In a tech support scenario, Dvorak is always troublesome. You sit down to type at someone's computer, and random characters start popping out.

People make this argument a lot, but you should optimize for the most common situation before an edge case. I am usually at my own PC/keyboard and I greatly enjoy my weird keyboard and layout. I can't type qwerty as well as I used to, but I rarely have to. (If I had to do it more I'd probably retrain it to make switching back and forth less troublesome).

It reminds me also of people who swear by using default settings in their editor or other programs so they can feel at home anywhere. Yeah, that's sort of a benefit, but I don't think it outweighs optimizing your workflow at your own machine.

I had a friend who used a split keyboard, blank keycaps, and a very odd layout (QGMLWB or BEAKL2 I think) at work. IIRC he said he kept a second normal keyboard at his desk for when someone would come by to pair program. This is sorta the inverse of your scenario. I guess he'd need to carry his keyboard to someone else's desk, or just type slower.

In general, even if it may not be "optimal," significantly non-standard keyboards (yes, Mac isn't quite like Windows) is IMO more trouble than it's worth.

It is a problem for any well-designed application with shortcuts. For they'd make the shortcuts often used to be according to the OS HIG, and a priority to easy to reach keys being also shortcuts often used (in that order). The latter is just part of learning curve (we all grew up with Qwerty, right? right?!) but the latter is an issue for any non-standard Qwerty (depending on how much they differ from Qwerty; which Dvorak does a lot but Colemak and Workman and Azerty and Qwertz already less so).

So what you say counts for any non-standard keyboard. There's always a learning curve. I tried going to 60%, now I settled for 80%/TKL and there are situations where I miss the other 20%, but my (vertical) mouse is in a more natural position. At least with Dvorak, all the physical keys are the same size as a standard Qwerty, so you could just set to Qwerty English-American and be done with it.

Tech support scenarios are most frequently remote, when both the IT person and the assisted user are typing on their own keyboards, sharing the screen.

In this normal scenario, the keyboard layouts do not matter.

"Tech support" when both people are in the front of the same computer happens more between friends or colleagues, when typing speed does not matter, than in professional corporate tech support.

This is called 'security thorough obscurity' - no one can mess with your computer in case that you forget to lock it (e.g. when you go make yourself coffee at work).