Tankspam.
Trying to do anything else but mass-spam attacking with your elite units almost always ends in failure. Sending small raiding forces into the enemy base may net you a few kills and a destroyed building or two, but no matter because within a few seconds the building will be reconstructed and 2 new units will be produced for every 1 killed. Stealth, hit-and-run, or any of the more interesting tactics didn't work well at all because every enemy will instantly know where you are once you attack and you'll get swarmed within seconds. Defensive emplacements can also see and shoot through obstacles (buildings), so cover is useless.
The only viable solution is to build up a massive force consisting of your best units and rush the enemy base so that you can demolish all of their buildings in one fell swoop. I found this tactic most effective on pretty much every map across all 3 campaigns. This means Mammoth Tanks for GDI, Avatars/Stealth Tanks/Vertigo Bombers for NOD, and Annihilator Tripods and Devastator Warships for the Scrim. Once you have your force, just box-select them, and move-attack on the enemy base. There's nothing like a cluster of 30 Devastator Warships spraying white stuff all over the place.
So it basically becomes a race to see who can build the most elite units the fastest. The superweapons are effective as well, so you'll want to build for those - Ion Cannon, Nuke, Rift Field. Nuke and spam. Repeat if necessary.
Some of the missions were poorly balanced. Some were laughably easy, others were fairly close to impossible.
High production values, but lacking originality, using the by-the-book RTS formula. CoH and DoW had more going for them. At least there was some strategy involved.