I do agree that Gandalf is a bit weak right now, but I think his Wizard Blast and Bubble Sheld are fine right now. Wizard Blast deals decent damage already, and it comes back incredibly quickly, especially once you are Gandalf the White. Also, it is a level 1 ability, so it really shouldn't be wrecking armies. And his bubble shield, in my opinion, is in a great spot. Comparing it to Thranduil's, it comes back much quicker, and most importantly, doesn't take up an ability slot. And even if the actual effect is weaker then Thranduil's, you also have to remember that Gandalf is NOT supposed to be a tank, while Thranduil is.
The way I see it, Gandalf needs 2 things right now. First, as you said, Word of Power needs its damage increased. It shouldn't be able to insta-kill armies of elite units, but basic units should take more damage from it then what they take now. And second, Gandalf should have his base health increased to 4500 instead of 3500. Gandalf's main problem right now is how much of a risk he has of dieing, and this increase would reduce that risk. Also, Saruman already has this amount of health, so I find it a bit unfair that Gandalf doesn't.
I agree that Boromir needs a buff. Right now, I think the best option would be to improve his Last Stand to where you can control Boromir while it is active, and it acts more like an invincibility, so instead of dieing when the effect ends, he will be left with about 10% of his health left. Other then that, I'm fine with Boromir.
Beregond (Not Berethor, the guy from the Third Age game
) indeed does more then he is worth when he is level 10, but I think this is countered by him being underwhelming until that point, so I am fine with him.
Denethor was changed due to unfixable bugs when he was on the Citadel, so that won't change.
Thanks for the feedback on Gondor. It is always good to see people take interest in posting about Balance