I have a Stg-556. I think it's a plus that it uses AUG mags because it's harder to do mag changes with a bullpup and that 40% increase in capacity over AR mags makes up for it IMO. MSAR also made the E4 which uses STANANG mags.
RatWorx USA makes parts for MSARs that fix a lot of issues like trigger pull. I imagine they make the same stuff for AUGs if the parts aren't interchangeable.