Ow man, I remember that. We had this super nifty rendertarget interaction thing from UDK (at the time anyway), which became completely unusable in UE4. Sure, we had very little access to that functionality (in that we could only make a water surface with some basic interactives, and that was about it), but I missed that when I started using UE4.
The UE4 documentation tells me it took them until 4.13 to finally put that back in. I really hope WPO in Nanite implementation will be a bit faster. Or that we at least get something of a solution instead before 5.13 or something.