My understanding of the paper is that it's pretty damn hard to get a new theory of gravity, other than general relativity. You might try and it might be ok at t

My understanding of the paper is that it's pretty damn hard to get a new theory of gravity, other than general relativity. You might try and it might be ok at the perturbative level, but once you go to the strong field regime you are screwed, with development of instabilities. Their conclusion is applicable to a large class of modified theories of gravity.