I used those round pin plugs for years growing up. Sometimes round plugs, but much more often the flat / stretched hexagon versions. And the outlets they plugged into did okay, but not great.
The three plug grounded plug in the US is also retained very well. But because it is optional, all of these wall warts skip it in favor of less hardware.
The UK version has the “active grip” advantage as well even when you plug one of those two prong round plugs into it. You do have to stick something into the ground opening to uncover the bottom holes, but once you are plugged in and release the ground opening it positively holds the plug in place.
I grew up the middle east - we had a very odd mix of outlets, plugs and transformers in the house to make everything work - I think I have used and wired most of the european, UK or US outlets available at some point.