Design flaws? in an ARF? nahhhhhhh........
The most fatal one is the bulkhead that the wing tab goes into, that holds the fuselage to the wing (or wing to fuselage if that's how you prefer to think of it) is ONE PLY OF LITEPLY! Or it was on the two Sukhoi's I've assembled. Both of them started to crush that bulkhead AND the two layer LITEPLY wing tab that fits into that little slot in the bulkhead.
I doubled around the bulkhead slot with real plywood and doubled the wing tab with real plywood.
If you haven't had that connection start to crush and crumble, there is a good chance you will. First thing you'll notice is that the front of the wing has some play. Once it has some play, it goes to hell in a handbasket pretty quickly. With my first, it took two flights the first time out. The second one started life with the doublers. No play yet, and that sucker sees some hard flying. Sometimes I think some of these flaws are just ignorance, but sometimes I think they're downright criminal. Fraud comes to mind.