As it is now ? As it's always been, actually. I've been whining about Guardsman promotion being detrimental since before Magister Cultuum even thought to create a FFH2 modmod. The Guardsman promotion crappiness however is not to be confused with Assassins and the Marksman promotion. Honestly I think that no cheap unit like Assassins should be allowed such a powerful promotion for free, but then again it must be an american legend that if everything is out of control or unbalanced, it will balance itself off pretty well. We can see proofs of how well this theory works in our everyday life, when we read of random people slaughtering other people with fire weapons even a soldier should have a hard time to get a hold of, or when we read the reports of the Stock Exchanges accross the world.
Are you seriously implying that the FFH2 team has done a bad! job at balancing an inherently unbalanced game by default? (Its just not possible to balance 21 distinct factions in a flawless way. At least not in a sensible time-frame if it would be worth even a lot more than it most likely is.)
Perfect Balancing simply is not a huge priority here to begin with (It has been mentioned more than once that it isn't by the team. The focus is much more on replayability. And there is some! contradiction between those 2 goals, isn't it?). And it still works out in a good way imo (perhaps not all so much from a competitive multiplayer-perspective. Given. But one game has a hard time catering to every ones preferences. FFH2 already is nothing short of outstanding in that respect by most accounts.

).
Given that set of priorities in my view its hard to find a better approach to find some! sort of balancing that doesn't utterly destroy the premise (maybe I'm just uncreative to come up with one though... Feel free + encouraged to make any recommendation you come up with. Its likely to be better than what i could come up with.

At worst a good one will very likely make it into a nice modmod, enhancing the game for many who would like more compareative balance for multiplayer / other reasons.).
For players who utterly dislike minor or moderate unbalances there are plenty of solid choices of games which are streamlined with that in mind to a large degree... (provided modding FFH2 / or finding someone to do it for you just isn't your cup of tea.)
Also assassins might! be really powerful but they are not the only thing and there are many other ways to solidly win a game even earlier. (you might not like that approach, but for me it works out fine. Even with the sidar which have a very powerful recon-line / assassin UU)
First and foremost simple warriors. Especially with copper weapons but even without them.
Assassins can't hope to compare in relative terms given the time both arrive in a given game (yet you likely hear a bit less complaints about warriors than about assassins from the balancing-crowd... At least in my perception. Which surely may be well off the mark.)
The point is not that its balanced as well as possible, but that balance doesn't auto-translates into a more fun game and thus sometimes has to take a back seat (and here it does rather often. Very luckily imo

).
Hyper-balance is horribly overvalued. As can be seen in Civ4:Col where taking pains to trying to balance and streamline everything (and include as many rubber-band mechanics as possible) + inadequate time to test and polish the game (which led to some serious oversights) has led to a broken and rather unfun game on release (it may still change a lot!/ have been changing to some degree already due to some excellent mods and patches but the initial release just strongly leaves that impression.).
I'm sure for multiplayer it plays out rather well in comparison (and seems to have been balanced with much emphasis on that aspect, costing quite more than a bit in terms of single-player experience.)
Still (even if it will work out in a good way) its an utterly different kind of game...
Also comparing a fun game to very serious incidents with grueling consequences in real life is just far-fetched and inadequate imo.
At worst here the game is not fun for some people because of it (and its not that its hard to modify the power of Assassins by one point for example... So not even that one is an un-changeable consequence from a players perspective).
So perhaps scale that kind of comparisons back, a bit?
So, no, FFH2 not all out balanced. Its still an outstanding game by most accounts in spite of that.
A better point to start might be to ask if the game is really made bad by assassins (as compared to the old meteor swarm spell in therms of balancing of overall spheres / spell chart) / impede on the experience in a serious way for most players. I for one can't see that.
Because you can bet, that a nerf would seriously piss off a lot of players (at least if its done to a degree as initially planned for the release of 0.34. Which was to make them national units with a limit of 4. That part even made it into the initial changelog. And thus nearly made it into the game... I'm sure some players might like it. I sure wouldn't. And strongly so.)