so i'd say overall they seem to be useful enough
Maybe, but only if you don't already have a few champions you hired at lower levels, or don't have the ability to hire henchmen. The high-level heroes generally cost 1000+ gold each, and all they bring is some equipment and maybe a unique trait. Once I have two or three champions, I have all the heroes I'll need for the game unless I really want a governor in each city, since the game economy is set up in such a way that you can't really afford more than a couple main armies (defined by me as "large armies usually led by a champion"), and unless you leave an area unsettled for monster farming you rarely (in my experience) have enough quests and random monsters to level more than two or three champions to a decently high level - at least unless you have access to quest maps.
By the point in the game where I can afford to hire one of the high level champions, I don't need them anymore because I've already obtained my two or three combat champions, and 1000+ gold for an extra governor, or for an extra spellbook for my sovereign, isn't worth it. Plus, the equipment the champions come with is, by and large, ignorable. There are a handful of special mounts, and one or two champions come with a special weapon that does extra damage per hero level, but most of them have at best a set of chainmail armor and a longsword (or equivalent). I can usually get equipment at least equal to that through quests and monster hunts, or at least gain enough loot to pay for purchasing that set of equipment (although I generally consider it wasteful to purchase anything beyond leather armor, a mount, a dagger/bow/magic staff, and perhaps a few trinkets like the Ring of Embers for a champion - once you can get the high level champions, you ought to be able to field decent trained units which can serve as much better meatshields than most champions ever could, and you can rush-train several of those for the cost of outfitting a champion in high-end gear).
I personally feel that champions should always have at least as many traits as levels, and that if they have a weakness they should gain an extra trait to compensate, or gain a special ability, unique weapon, or special mount. I'd generally consider the sum total of the equipment the champions carry is not worth any more than one trait, except in the cases of unique weapons and mounts, which might be worth a trait each - which should be taken out as a cost increase to the champion, or out of the traits beyond the one-per-level minimum. Sovereigns get anywhere from one to nine traits at level one (you have to have a profession, but aside from that you're pretty much free to do as you will). While I wouldn't say that most champions should be on par with average sovereigns (I consider an average sovereign to have one or two schools of magic, a profession, and four or two traits at level one, with no weaknesses - and the gauntlets of grazna, which are equivalent to most of a suit of leather armor, are only worth one point while most weapons are free), they should at least follow the same general rules as with sovereign creation - a set of level-appropriate weapons and armor is at most equal to one trait, and a particularly good weapon/mount/piece of equipment might be worth another trait, but no more, and you can't have less than one trait per level.
If I were to redo champions, I'd probably try creating them as if they were sovereigns drawing from a four-point pool, with an additional point for each level beyond the first, with a free blood trait and a free starting trait. Level appropriate equipment would be free, mounts and special weapons would cost a point each. So you'd see champions with up to five traits (up to seven with a weakness) at level one, and then one additional trait per level beyond that. Schools of magic would be priced as with sovereigns, and equipment appropriate to the next level group of heroes could be had for an extra point.
My view of level-appropriate equipment, as drawn from the champions currently in game:
Level One: No armor, weapon is a dagger or equivalent
Level Three: Partial suit of leather armor, and a dagger or equivalent
Level Five: Full suit of leather armor, and a battle-axe or equivalent
Level Seven: Mixed leather and chain/light plate or partial chain/light plate, and a battle-axe or equivalent
Level Nine: Full chain/light plate (possibly full plate or masterwork chain), and an enchanted battle-axe or equivalent.