>>96504496
The area where Marines REALLY excel is in indoor fighting, CQB, and boarding actions.

This is also the area where the Tau tend to be weakest since they just have to rely on Fire Warriors for this job.

That said...Fire Warriors are still a good chunk ahead of most Imperial Guardsmen so they're not totally inept. It's more similar to Space Marines vs Tempestus Scions. The Scions are no Space Marines but they're the best non-augmented infantry you're gonna get.