GDI should have the stronger tanks in the game, because Nod is focused on stealth and speed. This is what C&C lore tells us anyway. It's very hard to balance this, but in the original Renegade, the artillery was extremely powerful that it made mammoth tanks and the MRLS useless. Only an organized medium tank push could break through an artillery barrage. If you never played the original Renegade you wouldn't understand how overpowered the artillery was. GDI could not move anywhere without taking splash damage, even inside their buildings. And your screen would be shaking all the time because of the artillery shells. It sort of made the game ridiculous because Nod had the best tank in the game and also had the stealth advantage. The devs changed this in Renegade X by making the artillery much weaker than before. In my opinion a bit too weak as new players have been complaining about how overpowered GDI tanks are.
As for the stealth black hand soldiers, we can only wait for the devs to fix all the glitched beacon placement spots and possibly make sbh more visible (the bloom and visual effects in this game make sbh much harder to see than before). Plus they can sprint now so it's easier for them to juke you. I agree it's overpowered at the moment, and many people have abused the glitched beacon spots to singlehandly win games.
Note that this game is a work in progress and the original Renegade was not perfectly balanced either. My general impression is that Nod can easily win maps where there is no defense because of sbh, and GDI will win maps where there are defenses because they have better tanks and they don't have to defend against sbh.
My suggestion to the devs would be to fix the bugs, make sbh more visible and beacons louder and more visible too, and buff the artillery. I feel this will make the game much more balanced.