To start with This topic is from my personal opinion and past experience in MO I will not say that my opinion is the best opinion feel free to comment and correct
After several games I played as america and seeing other playing it I think Riot trooper have pretty low tactical value in most situation (infantry only game is one of the exception)
First his most shining ability is being able to clear out garrison from considerable range which is good but there are better option to do this
Add 400$(from Riot trooper 1000$ cost) and you get Abrams tank which is same tier unit on tier 3 While Abrams can out range most garrison if micro managed well and also able to deal with other armor , infantry
and structure with its versatile weapon Riot can only kill infantry
In some cases if I want garrison to stay intact after clearing I just bring some engineer and repair them and your infantry is able to garrison them again
Second the amphibious ability is not that useful as Riot can't effectively deal with ship it can just help him clear out shoreline garrison or crossing water space but we have the Warhawk which can do both with addition of only 150$
Third on the army composition I found that Navy SEAL is more effective to mix into my tank division than riot While SEAL can't clear garrison without risking their lives Abrams as mentioned before could do that job and Seal is 350$ cheaper which is pretty big different on mass production SEAL also have Riot's amphibious ability and able to assist tanks on base demolition with his C4
Riot trooper armor and extra health don't help much while their armor allow magnetic weapon to be used upon
So let's wrap it up Riot tier 3 requirement and 1000$ cost make them a mid to late game unit and other unit also have ability to safely clear garrison
Only thing I found good about them is longer flashbang range than bulldog and usage in infantry only mode