The biggest drawback is that you have to rely that the manufacturer is ethical and isn't loading your shit with bloatware/spyware, which, lol. I suppose one benefit is that, theoretically, they can spend more time and resources on designing a laptop instead of licenses for windows, but I remember reading somewhere that Microsoft eats up market share by providing their OS for free for PC manufacturers. Is that still true?
I'm not currently looking for one, but I am curious about them. For my use case, it would be mostly light programming, web browsing, movies and videos, photo editing, and light gaming. Doesn't make sense for me to shell out $1000 for some spyware linux junk when I could just buy some used 2015 windows laptop and install a distro on it
Microsoft has OEM pricing for manufacturers but they didn't even let the One Laptop Per Child project have windows licenses for free.
I wouldn't really worry about whether linux is preinstalled or not. It's not hard to install yourself and cost wise the only thing to consider is if the end price is worth it to you for what you are getting. What you should do is research linux compatibility on the specific laptop model before you buy. A laptop that either has linux preinstalled or has an option for linux preinstalled (eg: you find the laptop model with windows installed for a good deal but it is sold elsewhere with linux preinstalled) then you can be confident that you won't have any compatibility issues and the tech support won't refuse to talk to you if you mention linux.
Thinkpads and the newer dells are generally good for linux with a strong secondary market. If you want to go the linux first route then I've heard good things about framework or system76.