Porcelain has very good temperature shock resistance, stoneware quite good, earthenware bad. Your standard mug should be stoneware and take it just fine. There's even stoneware pots.
The issue is rather that you shouldn't use standard electric stoves with too small pots, on gas I guess that's half-sensible but you'd be left with a charred mug that's way too hot.
On that note, as someone from a commonwealth nation, I was deeply appalled during the height of the pandemic when kettles couldn't be purchased here as they weren't considered 'essential items'.
One reason that some Americans microwave water rather than use a kettle is that our electricity is half the power of UK electricity. It takes a lot longer for an electric kettle to boil here. That said, I do use a kettle when boiling water for tea.
When I went, if I ever saw one it was the equivalent of those cheap travel kettles. I think the average person there just doesn't use it enough to justify getting a good one.
We have a Zojirushi. 120V does limit it somewhat, but it's fine.
The water in our area of country is also hard as shit. We have undersink RO now, but before then, mineral buildup in the kettle was bad. Crusted like concrete if we didn't stay on top of it.
...softeners are essential in aquifer country; our zojirushi served us well for a decade but after our whole-house filter blew out a couple of years ago i'm starting to see iron deposits despite the softener...
US water softeners are usually only on the hot pipe. They tend to add sodium to the water, and it's not recommended to make it your primary drinking water source.
...nope, we installed ours on the full water supply: it's essential here or plumbing fixtures will fail...a properly designed softener won't add significant salt to your potable water since the brine flushes clear after each recharge cycle...
What a bullshit excuse. I'm in Canada with exactly the same 110v power, and it takes very little time to kettle water. People say this all the time as some sort of justification, but it just isn't.
Wow, that was a little strong given the subject. I'm not sure what I did to deserve being cussed at when I was just talking about electric kettles. Especially since I said I do use a kettle myself.
Do microwaves have some magic efficiency trick that lets them produce heat faster from the same exact energy? Like, how do they manage to be more than 100% efficient?
Microwave magnetron efficiency is around 65%. Since a kettle turns electricity directly into heat, it's basically 100% efficient.
A caveat is that microwaves will heat water directly and won't lose as much to its surroundings. This is similar to why induction stoves are more efficient; they're less efficient on paper than direct electric heating or burning gas, but they heat the thing you want in a more direct way.
Even so, a microwave isn't great for this task. If you're short on space and don't want even a small travel kettle, I can see why you'd take this option. Otherwise, no.