1: How would you have issues with the cooling system? I'm not too versed into water cooling systems so I cannot speak from those, but it's just a fan with a heat sink that you plug into the motherboard. Unless you go about shoving the connector in the wrong port (which shouldn't even be possible, you have internet tutorials and the motherboard manual which TELLS you which components are meant to be plugged where)
2: Software issues? I've never had one that correlated with the hardware itself, when I did have one it was an issue inside the operating system that was fixed INSIDE the operating system. Once again, user error.
3: I've never had BIOS issues, ever. Don't mess with delicate pieces of the motherboard and don't constantly flash the BIOS and you won't have an issue. User error.
4: I've had this build for nearly 4 years now, stuff never "started breaking faster" that's an issue on the users part. The only "breaking" I've done is taking out a part and putting a new one in (which is just like legos except a bit more advanced.) Unless the person in question happened to get a component that was broken upon shipment or had some sort of malfunction already, then this is user error.
Most issues are user error (Improperly plugged in a component, bended CPU pins, etc...)
There is NO hassle of continuous maintenance. Opening the case and dusting it off every once in a while is some serious maintenance. Tying cords in a certain fashion to allow more optimal error flow is definitely "serious" maintenance.
Fact is, a prebuilt computer is a waste of money (unless it is a laptop, even then you'd have to pick a good one from a good brand, such as MSI, Lenovo, Asus, NOT HP and especially not Apple), a custom built computer allows you to pick exactly what you want, give you the hands on experience (which is easy, building computers is one of the easiest things I've done...) and you learn the parts and what issues may arise from such parts so IF (and that's not a very big if) something goes wrong you know HOW to fix it, instead of shipping it to a corporation who will charge you lots of cash for something that can be fixed for half or even for free, then you have to pay them, then you have to wait. When if the issue happening is something that can be fixed by the user easily it can either be done for free or almost free (compared to shipping it to the corporation.)
You don't have to be an expert to build a computer, you could be a complete neophyte whose never seen the inside of a computer, I was and I did just fine, and many who was in the same situation I was have done just fine too. There are hundreds of tutorials on YouTube, there are hundreds of boards to console, you can get advice from someone from the other side of the world almost instantaneously. Unless you have a very good reason (being new is not one) then building a computer is a very viable choice and is very easy, cost effective, fun and will teach you many things. Not to sound rude, but it seems you have little idea of how easy building a computer is, how easy it is to fix one, and all around oblivious to building a computer in general, how easy it is to fix minor problems and how to avoid problems the bulk of your statement capitalized on.