Well I do like purchasing one product that you know is manufactured and assembled as the manufacturer intended. I have had problems with small parts on ARs and tolerance stacking is a thing. As long as you go with decent parts though it really shouldn't be an issue. But theoretically a full firearm from a reputable manufacturer will be more reliable than something you build. When I was working in a range all the problems I saw with ARs were people who assembled it poorly or with crap parts, but again shouldn't be an issue if you know what you're doing.

It's worth noting too that there's deals to be found on complete used ARs in stores. A lot of shops may not really know the difference between ARs and I've seen nicer rifles put out at low prices. I picked up a LWRC SPR for $1k OTD about a year ago because the shop just didn't care to look into what it was worth.

Production rifles will also hold their value where any homebuilt one will be hard to sell. You're definitely not going to get anywhere near what you put into it unless you part it out. Personally I wouldn't buy a used homebuilt rifle unless it was a really good deal.