- Incompatible & proprietary motherboards, coolers and power supplies
- Bizarre part choices, like severely underpowered cooling and power delivery or using just 1 stick of RAM when 2 sticks would be significantly faster with about the same amount of money
- Windows installation filled with unwanted software and/or outright malware, like McAfee, which can have a double-digit % effect on game performance.
- The purchase process can involve dark patterns, like cleverly hidden support contracts with a monthly fee
The PC hardware channel Gamers Nexus has been reviewing prebuilt systems for past few months [1]. None of the reviewed computers have been great, and the ones from big manufacturers (namely Dell and Alienware) have been the worst of the bunch.
You pay more for less than if you did it yourself and it isn't hard to find where they've cut corners to lower their costs.
But also critically the people buying pre-builts are obviously not interested in doing it themselves. That is why the market exists, right? So like its fun for more serious people to dunk on them in Youtube videos or whatever but I think its something pretty overblown.
Also if you're really willing to spend some money, there are plenty of Boutique vendors (Falcon Northwest) that will gladly sell you a capable and well-built computer.
Price vs... price. You can nearly always build a more suitable PC for cheaper(potentially with the extra steps of paying someone to build it for you). Though recently I've heard of a distortion where apparently due to high GPU prices, some prebuilds' value was more fair.
Dell likes using proprietary parts (motherboard, psu, case etc.) that restrict upgrades. Also they tend to be overpriced, to have bad cooling performance and build quality.