[conistonwater]

I don't know about your radiative exchange example, but it is often possible to reparametrize the problem. For example, sometimes you can replace the parameters (x,y) with (s,t)=(x-y, y), which would get you further: x^2-y^2 = s (s+2t), which is stable at s=0, and if x>y>0, then it's stable everywhere. This can shift the region of instability to somewhere far from typical parameter values. Otherwise, yes, this isn't solvable.