In the real world ratcheting ends are used to break fasteners loose all the time. Shouldn't be done, but it is done. Not sure what dated info you're going by but Matco fully supports breaking loose fasteners with their ratcheting box ends. They even went so far to make this silly vid touting the strength of the ratcheting mechanisms and even though it isn't stated, considering all 13mm wrenches took well over 100 foot pounds, in my opinion, there should be no hesitation to use them to bust something loose.
Flex **** when you're trying to snake the wrench someplace where you cant fit your hand since you generally need to support them with some pressure to stop the flex end from flexing as you try and fit it on. Especially frustrating in rusty areas where fasteners grow substantial rust scale which means any box end or socket needs to be persuaded on. This can be done at length with a fixed end, but not a flex.
Flex have their place but if they were my only set of ratcheting wrenches i'd probably of torn my hair out from frustration long ago.
