I have raised Cornish X, Freedom Rangers and a "heritage bred" (Faverolles). Granted I have only done 60 Rangers over the past couple of years, but none of mine ever had any extra fat. And certainly none were bony or tough. All of them were well fleshed. The amount if breast meat harvested was similar to what I would get from an 8 week old Cornish X, though it has a little more chickeny flavor to it. The main difference was in the dark meat. The Rangers have longer legs. Combined with the longer growth period it resulted in more flavorful dark meat (in my opinion). I like dark meat, which is why I prefer Rangers. However this fall I wasn't in the mood to raise 25 of them so I am going with Cornish X from a local feed store. They were a good deal given they were already 10 days old. It is amazing how quickly they reach butchering age.
To the OP, true heritage breeds do take a long time to reach maturity. For most people it doesn't make sense economically to raise them for meat. I just happen to have extra cockerels and pullets from my spring hatching, along with older unproductive hens that are culled in the fall. While they do not provide a tremendous amount of meat, they do produce outstanding stock that the meat class birds cannot, and they are great canned for later use.