Today why Mechanic Assault Doctrine is so popular, I guess, is because of the problems I mentioned. But that would be a doctrine disadvantage. Moreover, Mechanic Assault does not work well against UKF, as Assault Grenadier does not have panzerfaust, Stug E could not penetrate middle tanks, British Sniper could easily wipe 250. Moreover, if Centaur AA comes, what would I do? British infantries would help Centaur clear Paks.So I could only use Stug G or Pz4. But that would make no sense to use Mechanic Assault.
I find mec Assault is a really great doctrine against UKF for you can skip T1 with AGrens can do a really great job against IS and now that you can make MG's from your HQ is a big advantage.
The build order I usually use with mec Assault is MG-AGren-MG-AGren-AGren-Battle Phase 2-Pios then the rest is up to you