Easy to mispolarise, not earthed, not safe when partially inserted.
I've never given polarization any thought and never had any problems, so I don't buy this being an actually a problem in practice.
> not earthed
NEMA-1 aren't earthed, but NEMA-1 sockets have been forbidden since the 60s, so that fails the "unchanged" part of coldtea's assertion. NEMA-1 plugs may still be used on Class II double insulated appliances, which is fine. And NEMA-5 does have a ground.
> not safe when partially inserted.
I've never worried about it, never been hurt by it, and never heard of anybody being hurt by it. This is a "problem" that I often hear described by Europeans online but it's totally alien to me. Please remember that we only put 120V through these things, which is substantially less nippy than 240V.
Just as my anecdotal experience: I've shocked myself at least twice because my finger slipped over the contacts while I was trying to plug/unplug something in the dark. I have of course learned my lesson and am much more careful when plugging stuff in, but it's a relatively easy mistake to make if one is tired or not paying attention.
Yes, I got shocked because I was stupid and careless. But a connector that everybody uses every single day should be idiot-proof -- it should not be possible to kill myself because I plugged something in carelessly.
The US has the "excuse" of being the first domestic power distribution system -- other countries have better standards because they were able to learn from the design flaws in our plugs. And our split-phase 120V system is (as far as I know) a great idea that's way safer than direct 240V. But there really are a lot of things that the NEMA plugs leave to be desired. I second the recommendation by a sibling commenter to check out Technology Connection's videos on the subject -- I found them to be entertaining, nuanced, and highly informative.
Wife has same and got a zap. I never have and my own fingers are a bit pudgy. This happening never even occurred to me.
Are the US plugs actually original? The UK ones are on their third generation (originally various proprietary or light-socket-based things, then the three prong round ones found in very old buildings and specialised industrial applications, then the three prong rectangular ones from the 60s).
> And our split-phase 120V system is (as far as I know) a great idea that's way safer than direct 240V.
Eh. They are both safer or less safe in different ways. The US system requires about double the current for given power, so is less forgiving of marginal wiring.
That's probably because a lot of equipment is double-insulated now so polarization doesn't matter, and manufacturers are better at getting the polarization right. It's usually when someone (incorrectly) replaces a cord themselves that you run into issues, which doesn't seem to happen much nowadays, the lamp with the bad cord goes in the trash. Though I've certainly run into home outlets that are wired backwards.
Probably the most likely place you'd run into problems with incorrect polarization is when changing a light bulb. If the polarization is wrong, the outer threaded part of the socket will be live, and you've got a good chance of contacting it while changing a light bulb. (which itself is becoming a rarely with long lasting LED bulbs, or LED lamps without changeable bulbs). Though this is probalby more the fault of the Edison lightbulb socket than of the polarized plug.
I would be terrified of the american plug. I played with one when I was over last time and there was over 5mm of exposed live connectors. That's bonkers.
In my experience polarization has only became relevant once: when mounting a dedicated socket for a gas furnace. For some reason these don't like the live and neutral being reversed and go into an error mode when that happens.
Other than that, most wall sockets in my house are dual, which means polarization in the top socket is reverse of that at the bottom and it has never caused any issues.