Is there any real downside to buying a prehung door from Lowe's?

I swapped my own back door about three years ago and went the big box route, and it's been fine. No drafts, latch works, finish still looks decent. Your brother-in-law isn't totally wrong, he's just overstating it for your situation. alot of the slabs sold at the big stores are the same doors the lumberyard sells, sometimes literally the same manufacturer. The difference shows up in prehung assembly and in the high-end stuff. If you were doing a fancy front entry with sidelites, sure, order through a dealer. For a back door on a 90s house? The stock prehung unit will do the job for years.

One thing I'd check before you buy: look over the actual unit in the store rather than taking a sealed box. Prehung frames get assembled and sometimes they're a little out of square or the weatherstripping is pinched. Pick one with tight corners and a frame that doesn't flex when you push on it. Also check the jamb depth against your wall thickness. If you've got 2x6 walls, the standard 4-9/16 jamb won't cover it and you'll be doing trim gymnastics or ordering an extended jamb. As for the old frame coming out, plan on it being a fight, just not a huge one. The door itself is easy. The frame is nailed or screwed in through the jambs and ussually caulked or flashed to the siding, so I cut the jambs in half with a recip saw and pried them out in pieces. Expect some rot hiding under that soft bottom corner, so check the sill and the subfloor underneath while it's open. If the framing shifted like you said, take a level to the rough opening and shim it back plumb before you set the new unit, otherwise your new door will latch badly too. Budget a whole day, not a morning. The threshold and exterior trim are the fiddly parts, and that's where the drafts come back if you rush the caulking.

I did this last spring and the thing that almost got me was the swing direction. I stood there in the aisle holding the door handle like an idiot trying to figure out which side it opens from, because aparently you order it from whichever side you pull it toward, which made zero sense to me til the guy walked me through it. Double check that before you load it in the car or you'll be making a second trip like I did. Also mine came with the little cardboard spacers screwed across the bottom to keep it square in transport, leave those on until the door is actually set and shimmed. Do the ones at the store usually come with those, or is that only on certain brands?

First post here so hopefully this is useful. I went the cheap prehung route for my side garage entry last fall, and the one thing that saved me was buying it in person instead of ordering online. The first unit they loaded for me had a cracked mullion or whatever the corner piece is called, and swapping it at the desk took five minutes. If it had shown up on a truck I would have been stuck rescheduling everything. So whatever you buy, factor in that you might need to make a second trip, which kind of strengthens the case for the store run over the lumberyard order anyway.

The frame shifting thing is what I'd dig into before buying anything, honestly. Mine did that and it turned out a downspout had been dumping water right by the corner for years, which is why the wood went soft in the first place. Fixed the drainage, let things dry out, and the new door's held fine. If there's no obvious water source, it's probly just settling and you're good to go.

Also for what it's worth, I'd skip the wood door and grab fiberglass if they've got one in your price range. Won't rot on you if the caulk fails someday.

Been hanging doors since before prehung was even a common thing, when you bought a slab and cut the hinge mortises with a chisel on site. Trust me, a stock prehung unit is a luxury compared to that, and I've put in plenty of them that lasted twenty years on rentals. The brother-in-law is comparing furniture to a weather seal, that's all.

One thing I'd add though, check that new unit over before you set it, beacuse the factory sometimes leaves the hinge screws loose or the weatherstrip glued crooked. I pull the middle screw on each hinge and run a 3 inch one into the stud, that's the one that carries the weight over the years.

Update: walked around the outside of the door today after that point about the frame shifting. I don't have a downspout issue like that poster, but the soil right by the corner does slope toward the house a bit, so water probably sits there. Gonna regrade it before I order anything, otherwise the new one rots the same way. Also triple checking the hand of the door this time, that aisle story is going to be me.