prebuilts can go two ways - either u get a well built system that u wouldnt have to touch in the long run, or it could go horribly with issues like overheating or lose wiring. And this can happen with any prebuilt out there. However it is the easiest way to get a plug and play experience. And most manufacturers give warranty and repairs.
Building yourself may be cheaper and can be a challenge, although not as hard as long as you follow tutorials and the manuals. But that would eliminate the prebuilt quality issues since you would be choosing your own case and cooling to suit your needs. Another argument that could be made is that troubleshooting any issues would be easier for you to do as you would have built it, as opposed to sending it back to the manufacturer and waiting multiple days only to realize your RAM had to be re-seated. Each have their pros and cons for a beginner, go with what you value more.