What..? Gilden is one of the strongest factions, imo, and were by far my easiest win out of my 6 games (conquest victory is the only way I play.) In fact, I would say they are the single strongest, as the Life tree is by far the best for the way this game plays, and Gilden gets it from the start on top of everything else (Fire would seem to make a lot more sense thematically, but w/e) - though Pariden was a close second.
I would say the wraith faction is definately the weakest, from their lack of healing, and due to unavailablity of the "design heavy mounted troops with the best spears you have researched and all the initiative artifacts and perks, cast Heart of Fire and the Initiative enchant on high essence Fortress, support with a few mage units (which are far more effective than any real spells), enjoy wiping everything off the map in 2 turns or less" that all the other factions devolve into. Quite disappointing, really - the way Heart of Fire works and the fact that the AI is too dumb to stack initiative makes the way games play out extremely repetitive and lackluster.
Tarth is bad from a different perspective, imo - they aren't really that much different than anyone else. Their archers don't come any earlier and are just a tiny bit stronger than normal - and that's pretty much it for them as a faction, aside from their scout bonus, which is sort of nice while expanding, and staffmages are far stronger than any archers no matter how you slice it. I can tell you that plinking things for 5-6 damage for most of the game if you try to level your sovereign as an archer is pretty disappointing, but mage champions aren't really much better off in tactical with the weakness of combat spells aside from Heal, Graveseal, and Haste/Slow, and at least the Tarth sov gets Heal. In general, I find the faction differences in this game quite disappointing, especially considering Kael is working on the game - as I said, no matter who you are, you end up with the same exact units/strategy, and that strategy just happens to be what Gilden is designed for.
The Gilden golems are great for cheap (crystalwise) city defenders, btw. If you really want to take advantage of the faction, make your main unit-spewing Fortress on any tile with 3+ essense, regardless of other stats - use Gentle Rain to get city levels (OP spell) and the enchants I mentioned (Heart of Fire and the Air initiative enchantment), plus the HP/Def ones if you can fit them, to buff your units astronomically. Of course, this strategy is the same one everyone uses, but Gilden's tougher units get even more of a benefit.