I picked out this card as being one of the hardest cards to use from the Against the Shadow cycle, an assessment which I think few people would argue with. Apart from anything else, I think it's incredibly difficult if not impossible to make this work in a single deck. It's really something you want for a multiplayer setup. One of the other things holding this back which I really wish was otherwise is that this only lasts until the end of the phase. If it was the end of the round, this card would be a lot more usable, because you could play it once and benefit from boosts to willpower and combat that round. But alas it is not to be.
To delve into this further, since this allows crossover between the Gondor and Rohan traits, we have to look at the things that can actually benefit from the Rohan and Gondor traits. The immediate thoughts are Boromir and Visionary Leadership. Especially the latter since Rohan has a lot of good questing allies anyway, so giving them all +1 can definitely make a difference. We could then stack it up even further by also playing Astonishing Speed and letting that boost go back to the Gondor characters as well. These seem to me like the biggest benefits you're going to get out of Mutual Accord, but there are some more possibilities. Adding in a Sneak Attack Éomund would allow you to go all out and then ready everyone afterwards as well. On the other hand I wouldn't recommend We Do Not Sleep - Sneak Eomund is cheaper, and if you want more readying options, for the same cost in the richer sphere you could pay Grim Resolve instead. So Mutual Accord can get us some big quest pushes. That's certainly not irrelevant, but can it do anything else? Well, it can do a bit for you in combat as well. I mentioned Boromir, and applying that boost to some Rohan allies could help kill things, and it's cheaper than For Gondor! Which could itself be used with Mutual Accord if you really want more , though a global boost is more likely to be really useful in a Siege-questing situation than for actual defending. If you want to get into some slight shenaniganry, Mutual Accord would allow your Gondor heroes to benefit from Forth Eorlingas!, though that card has some difficulties of its own - still, Faramir at least could work with it. Alternatively it would also allow them to benefit from Charge of the Rohirrim, which is perhaps more applicable, though there are unfortunately a limited number of Mounts you might want to/be able to put on said Gondor heroes. The final category of benefit you could gain is in the janky combo territory - you could use Mutual Accord as a cheap means of gaining access to cards you usually couldn't use. Just like you can use A Good Harvest to get access to out-of-sphere cards, you could use Mutual Accord to get access to out-of-trait cards. I'll note here that if you just want the trait you could equally play In Service of the Steward or Nor am I a Stranger as applicable, but if you start with one and want the other then Mutual Accord is a cheaper option. So what can you get access to? Well, Blood and Fire on the Gondor side of course, but for those you probably want Steward of Gondor to get the resources so MA is not required. Sword of Númenor? Sword of Morthond on a Rohan ally in your Outlands deck? Visionary Leadership because you built a Gondor deck with no Gondor heroes and no Steward? These sound weak, but the Rohan side is where the money's potentially at, especially since we don't even need a starting Gondor trait with Steward as a possibility. Getting Rohan allows you to attach Spear of the Mark, which isn't that great but could have applications (and remember it can go on allies); Snowmane, which is getting better; Rohan Warhorse or Firefoot, already accessible to of course but maybe you have a non- attacker, and either an extra attack or overspill damage could go well with Gondorian Fire; and of course probably the biggest reason you might want to throw the Rohan trait onto a character, Herugrim. Perhaps not the best fit on natural Gondorians, since they're not blessed with an abundance of , though Visionary adds a bit, but when you widen your scope and consider that you could use MA to put Herugrim on any hero once they have Steward attached, there's some impressive power in that as an idea.
The big point to note however is that all of these are multi-card combos. The plus side is that some of them, like Visionary Leadership, are cards which you already want to get out fast so you try and make sure you get them consistently, but they're still multi-card combos and thus inherently less reliable than many other options you could include instead. So, useful in the right context? Sure, absolutely. But it's really not a card you can just drop into your deck and expect to work. You have to work for those benefits. For an example of a deck pairing that did try to make it work (and I'm told there was some success), look at this fellowship by CotR's Sean: ringsdb.com