The tight joints at the chain stays and especially the seat stays are tough! You don't have a lot of room and if you didn't already, you'll certainly have a new respect for the guys who do this well. From a design perspective, this area is full of opposing priorities. If you want a big, fat tire in there, you need to move the chain stays outboard, but you don't want to get too close to the edge of the BB shell. If you need more clearance for your chainrings, you need to move the chain stays inward, but you don't want to get to close to the middle! You're miters will be extra tricky and it becomes pretty tight for a good clean weld. Of course the shape and size of the chain stays add a whole other element. Fun stuff!