Well it was inevitable that I'd review this card at some point. A lot of people are very negative about this card - unfairly so in my opinion, but even with that said this card is used so rarely that I have occasionally forgotten that Warden of Arnor is the name of a card as well as my blog, and have to remind myself when someone actually mentions the card.
Let's first look at the issues with the card. It only attaches to Scout heroes - which at the time it was released was only Idraen. Haldir of Lórien came in the next AP but suits this card less well since he quests less. Since then we've had Lanwyn, Elfhelm, Argalad and Legolas added to the ranks of Scout heroes. Argalad is less likely to quest than use his ability unless you give him some action advantage to do both, and Elfhelm being versatile could quest or not (might well end up defending since with a mount his ability would boost his ), but certainly we have more options now. Idraen is probably still the best fit though, because she naturally synergises with location control. The other downside of Warden of Arnor is a minor one - you have no control over where the progress goes, so if there are locations which have negative effects for progress being placed on them then this card is to be avoided.
Now on to the good points. Warden of Arnor is cheap at 1-cost, and it boosts location control up a bit. This is not a card you put in as your only location control in general, though that one progress will in each case make the location 1 point easier to explore when you travel to it; but no, this really is a card you include alongside other location control, and there it works very nicely. Consider the most obvious example, Asfaloth - since the release of Asfaloth a large majority of locations have at least 3 quest points to ensure they cannot be instantly explored by Asfaloth (attached to either version of Glorfindel). But if a 3 quest point location is the first location revealed when the Warden of Arnor is questing, then you're back to being able to simply nuke that location before quest resolution. A similar principle obviously applies to The Evening Star. On the other hand, if you're exploring locations in the staging area by means of Northern Trackers and/or Rhovanion Outriders then Warden of Arnor will let you explore the loction in question one round faster, which is not insignificant.
One progress every round (assuming you consistently reveal locations) isn't much, but the card only costs 1 resource. For that same 1 resource in the sphere you'd play a Snowbourn Scout and get just one progress full stop, plus a chump blocker. It might be argued that the chump-blocking is of more value than the one progress, and of course the Scout's progress is targeted, but how much of a difference does that make? How many more progress do you need before it becomes equivalent? And how many rounds is the game likely to last? How many progress will you get out of Warden of Arnor?
Warden of Arnor is not a card that's going to blow anyone away with how good it is, certainly not by itself. But in conjunction with other location control effects, and especially at higher player-counts where location control becomes that much more significant, it provides a vey useful supplement to those other effects, and can make a small but significant difference.