The only side you should have an excuse for losing with would be Vasari, on account of the distinct lack of powerful shield boosting abilities.
Shield boosting power is nice, but at the end of the day it all comes down to firepower. It's still pretty easy to roll an AI as Vasari, particularly in entrenchment with their lovely starbases. You can get away just fine with overseers and skirantra for healing as Vasari. Heck, the replay I posted is me crushing an unfair within 30 minutes with a well timed starbase rush.
You also have the Dunov shield restore ability.
Shield restore is overrated. It certainly works, but it's not worth sinking two capital ships to get the combo, IMO. My personal belief is to bring overwhelming firepower. If the enemy is reduced to space junk before they get through your shields in the first place, you don't need shield restore at all.
If his problem is primarily in resource management, advocating a multiple capital ship strategy is probably only going to dig him a deeper hole. Without knowing his precise difficulties, I would not recommend this approach.
Getting a starbase with maxed out weapons is fairly useless for the other sides in comparison .... Two Dunovs turn it into a beast without that.
If you have a fully upgraded starbase, brute force isn't going to take it down. If it's an AI, a fully upgraded starbase (especially one with fleet backup) is essentially an "I win" button that the AI will never beat, so this is a moot discussion. Honestly, I'd prefer to have some hoshikos, a few repair bays, a few hangers, and an Akkan (for targeting uplink so the starbase can hit Ogrovs) as support rather than the Dunovs. Anyways, bottom line is that you don't need the Dunovs to get this to work, and IMO you're better off saving the cash and spending it on firepower.
As Advent, it's even worse. No shield restore, and significantly weaker repair platforms
That's just incorrect. Advent's shield regeneration gives you 500 points of shield healing to every target nearby once every 20 seconds, whereas the Dunov's shield restore gives you 750 points to a single target once every 11 seconds. Advent's repair platforms are also equal to the TEC platforms until the TEC sinks money into 2nd tier upgrades. So while the Advent platforms are weaker, it's not significant.