AIO's and water cooling in general have significantly more failure points than air cooling, so all of the failure points should be something that you should be aware of. Cheaper AIO's generally aren't a amazing buy unless aesthetics are what you are going for since high end air cooling will often beat them for less or similar money. I wouldn't for instance put an AIO into a machine that the user isn't aware that it can and will eventually fail at some point, likely around the same point cheap fans will start to die.
None of this makes AIOs bad though, just different and more of an enthusiast part.
Most products usually fail either shortly after installation or shortly after the warranty expires. As long as you check for leaks and cracking every so often and avoid kinks or stretching out the hoses, you'll be fine. If it starts making noise, or your temps increase significantly you need to inspect it and check the coolant level, usually by shaking it and listening for it being really empty.
If the tubes fail outside of your warranty, you probably replace them provided the pump doesn't die from burning up in an air pocket for like days on end.