This is a card I've always liked but I've pretty much never used. It's a general problem with event-based boosts that generally you want permanent boosts to deal with all enemies rather than a temporary one which will get you maybe one or two enemies. I suspect that the most used -boosting events, certainly as a percentage of decks capable of making use of them, would be things like Unseen Strike and Khazâd! Khazâd! since they're free, as opposed to Charge of the Rohirrim or comparably For Gondor! costing 2 resources which may be much more difficult to save up. On top of that this card specifically obviously requires a certain amount of setup before it works at all.
One of the biggest limiting factors on this card I would say is simply the question of how many Mounts are going to be in play. This card is potentially much better with more players, if you have a bunch of Rohan characters in play who you can load up with Mounts to just kill everything. At the time this card was released it honestly probably wasn't that good because there really weren't that many Mounts you could use, and not all of them were Rohan Mounts - but as we get more and more Mounts, and particularly since the release of hero Elfhelm, this card definitely increases in potential.
Let's look at this in more detail. Some of the Mounts are generally or completely unavailable to Rohan, but Rohan Warhorse, Firefoot and Snowmane specifically reference the Rohan trait (so does Steed of the Mark, but we tend to ignore that one), while Arod can go on any hero; and then Armored Destrier, Steed of Imladris and Windfola all go on heroes of particular spheres. Meanwhile on the hero side of things (despite the event referencing characters, none of these Mounts go on allies), there are 11 Rohan heroes, 8 of whom have 2+ (including 2 different versions of Théoden). Of course, while it doesn't solve the problem of affording it, Háma would allow this event to be repeatedly recycled so it'd be more consistent as well. So there's definitely scope for a 2+ deck fellowship to get +18 or +21 out of this event without actually trying too hard. If you're willing to go a bit further with it, Nor am I a Stranger could get you some more eligible characters, and while 3 of the Rohan heroes are only 1 (including 2 versions of Éowyn), without even getting into the use of Éowyn's ability, there's just the simple fact that giving +3 to any character makes them a useful attacker. 3 is what we view as a good attacker, so +3 on any character makes them good whatever they were starting from, so if you can get your questers ready with Mounts attached then they'll be good attackers when you play this event.
The other limitation which you then have to get around is that with that (potentially) big a boost is having enough enemies to kill with it. So that's another thing which pushes this event more towards higher player counts, since there you're more likely to have enough targets for this to seem worth it. Of course then there's still the question of whether you can engage the enemies - so maybe this fits best into a higher threat aggro sort of setup where the enemies will generally engage you anyway once you're a little way into the game. Or of course you could just go all out for the weird Rohan combat options and double up on Charge of the Rohirrim and Forth Eorlingas! so you can kill all the enemies in the staging area - although it's more difficult to pull off, I'd say that may be a better option, because if you just stay below engagement costs and have plenty of you can leave enemies in the staging area for a while and once you draw the events you'll have plenty of targets for all those suddenly really high heroes you have. Also it's just really cool.
All in all, this is a powerful and fun card if you can get it to work, but getting it to work can be difficult. The best place for it probably requires co-operation in building or choosing decks to play together for a multiplayer game, with a Rohan focus and most likely using hero Elfhelm to just tie all of the synergies together. If you do go to the effort of building a deck (or more likely multiple decks) which make it work though, it can be well worth it.