This is an issue I've had with both Hitfilm 4 Express and Hitfilm Express 2017, silly me thinking that is was due to the 8-bit colour depth and buying Pro to try and fix it and finding out 8, 16 and 32 bit all look exactly the same in the end render, it's almost like harsh outlines and edges on things that shouldn't have them like shadows and smoke etc and places where there was very faint banding in the original is now amplified.
I do game videos and it just makes them look worse, I chose this over Vegas as despite those renders coming out looking almost idendical to the imported video when I tried the trial, I didn't like using the software.
I've tried all the colour depths, I've tried ever anti aliasing option, I've tried different bitrates and it always looks the same, is this just a Hitfilm thing we have to live with? I would have stuck with Express if it is, despite the nice extra stuff unlocked now.
also want to point out that it looks fine in the editing window too, just the finished render.
Sorry if it seems like I'm ranting, it's late and I'm tired after running render tests for hours.