Have you done it? Can you share some pitfalls? Were there things that just screwed up the plans?
The cost of parts is a huge factor for certified aircraft. I found this one to trip up many buyers of "cheap old airplanes." They get a deal on some older, less-popular model such as a Bellanca Viking, and then find that parts are scarce and expensive. They're shocked, as they didn't pay much for the airplane, so parts should be cheaper too, right? Well, are parts for 50-year-old Ferraris cheap?
Even older 172s with the flat-leaf main gear legs are quickly in trouble if corrosion is found on those legs, and that is very common. The underside of those legs were shot-peened to improve tensile strength, and they were also high-carbon steel that corrodes easily, and when pitting is found, and deep enough that it can't be dressed out and the leg re-heat-treated, you must have replacement legs that haven't been made for 40 or more years. Junkyard legs tend to be shot, too. Nothing lasts forever.
Many airplanes are full of horror stories simply because they have had drive-thru annual inspections. Lots of stuff that hasn't been opened up in many years, sometimes decades. Cracks, corrosion, corroded and/or frayed cables, seized pulleys are really common. And then a buyer wants a quick and dirty prebuy and doesn't want you to take the time to get inside stuff. His first annual tends to be a shocker.
Some old airplanes might have outstanding ADs that could sink whatever value is left in them. Sometimes replacing a single part can cost as much as the new airplane did originally. Think spar replacements, for example. Or the engine in it is so obsolete that you can't even get parts for it. I owned an Auster MK6 that had a deHavilland Gipsy Major engine in it, a British engine that had been out of production since 1950 or so. Good luck finding airworthy parts for it. Franklins are in the same boat, along with some old Continentals and Lycomings. Any airplane project that has more modern stuff in it will have a higher value just because parts are more available.
One doesn't rebuild airplanes to save or make money. One does it because he loves the work and plans to keep it. He's not going to rent it out to people who will abuse it.