The RNAV deviation scales introduced by Boeing make the picture much more clear if you are doing pure RNAV procedures, one glance tells you how much manouvering space you have at your current ANP and the required RNP.
And in general it is true that any RNAV approach deviation is linear unlike an ILS, however if you use boeings preferred method for non precision approaches (IAN) you will have a build in funnel similar to the ILS, even so far that you get GPWS call outs for glidepath deviation, that method is available on all 737 that have those nice RNAV deviation scales. The aim is not only to have the same procedure as for an ILS but comparable indication and deviation scaling as well. The same is of course even more true for GBAS-Approaches which for all practical means are identical to a normal CAT I ILS, including the funnel narrowing down when you approach the runway.