You know, this is a funny topic for me because I had a completely different opinion on this subject just a few weeks ago 
I like Railking; I really do. In fact - every engine I have is Railking at the moment, and only one (The F3 ABA set) is scale. As I've added Railking engines, I have become more and more aware of the size disparity - the F3's just seem enormous to me when they are on the layout. It never used to bother me, but as I have grown somewhat in the hobby, it starts to bother me more. When I see a RK steam engine next to a Premier steam engine on a dealer shelf, it gets really weird. I'm finding that even the differences in size with rolling stock has become noticeable. By all means, mix away if it doesn't bother you - for many/most it won't ever be an issue. Just be aware that some engines compress better than others visually - I've never been able to wrap my brain around a semi-scale GG-1 for example. The Railking PRR Turbine didn't do it for me either. That's just me; the engines are fantastic for the money IMO!
However...
I have begun to tire of semi-scale. The offerings don't excite me as much anymore, and I feel like I'm missing out on a number of things I'd really like to run but can't for lack of O-72 curves. I'm currently exploring methods to get an O-72 loop into my layout. My fear is that if I am successful, I will start moving more and more towards the scale offerings. This is bad for me financially, because as I mentioned above, mixing semi-scale and scale steam is really strange for me. I realize that there are plenty of Premier/Scale engines that do not require O-72, and that in a way that's not even part of this discussion, but for me it is because at the end of the day, it's about what I WANT to run, which is what this hobby should be
So that's kind of my take - Railking ROCKS, but beware! It may very well be the "hit" that pushes you over the edge to full-scale (pun intended...) addiction!
