All in one systems are built for use in a restaurant or bar environment - many are waterproof and dustproof so if you spill something on them, just wipe it off and no damage to the system.
It’s true that a lot of the all in one systems are quite low spec in comparison to standard business/consumer PCs and laptops. The reason being is POS software generally does not need high powered CPUs or lots of RAM to operate, and also the manufacturers tend to favor reliability and durability (waterproof, etc.) over having the latest, fastest hardware. It’s also not uncommon for hardware manufacturers to be providing 2 or 3 year warranty with the purchase of such systems, pretty much unheard of (unless at extra cost) for standard business/consumer PCs.
Most branded all in one systems are easily maintainable, so if a hard drive fails it’s easy to swap out and replace (in many cases, easier than on a standard PC). Of course if the motherboard fails that’s a different matter, but your average user isn’t going to be swapping out anything more than a hard drive usually.
For me, as our company provides POS solutions, we always recommend all in one systems unless a customer wants a specifically high spec PC. Reason is, they are actually far more reliable in the type of environment they are used (I have actually never had one fail other than user created damage). And for us, since we have an ongoing relationship with our customer, if anything does happen, it is very easy and fast for us to fix or even if need be have the hardware supplier fix.