If you actually take into account a bunch of considerations, it's an advantage but not an unstoppable one.
So, you can target anyone in 60 feet you can see. This is great if you are in the bushes and they're in the open. If you all are in the forest, 60 feet might be harder to get just from foilage alone. You might have to be closer.
Although there's clearly better and worse ambush locations, it's hard to predict exactly where people will go, unless there's an obvious route or land feature that they would likely cross, so even druids might not get the best location to do this at.
Verbal, Somatic, Material
You need a flame and a piece of iron. So already you need to have a flame you can access easily and not give away your position. You also need to be able chant/sing/etc. and gesture and not give away your position. Notice that the distance is a critical factor here.
So, red hot burning metal in an area full of woods. Yeah, those targets BETTER be outside of the forest, otherwise, you're setting fire to your own forest. Sure, it's going to take time to catch and you might be able to do something about it... but maybe it's gonna be a problem or the enemy will make it hard for you to put out the fire in time.
Are the druids working with other beings in the forest? For example, would the local Treants be ok with this being used? Often times there are other factors that keep people from doing effective things.
Smartness of NPCs
In reality, people aren't totally stupid. They're also not totally smart, either. We've got tons of examples of people using creative and effective solutions and cases of people doing really poor choices.
Is the druid coven organized and trained enough to have a set group of actions to do against invaders? Will they all bring covered lanterns and sneak around looking for threats? How often have they had to repel threats? I could see their patrols having a variety of spells, but unless they already know who the enemy is, I can't imagine they've all specialized for fighting people in armor, especially when they're probably facing more natural critters on the regular.
The thing is, as a GM, you can take a lot of things and use them to the maximum effect - because you already know the party's set up and loadout and you can assume that all of the NPCs will cooperate and act intelligently. You're not going to have one of the druids shapeshift into a bear, rush the party while everyone else was trying to set up for an ambush...thereby ruining it.
Context to consider
It's also worth asking if the party is up to that level of challenge - some players want mindless hack and slash and other players want to have to pull out desperate measures of clever tactics. If your group is not up to it, it won't matter how much you nerf a spell or ability, you'll figure out something else that is "too overpowered" because, yes, when used intelligently, it will do much greater devastation than otherwise expected.