I just don’t see it. Even with the longhouse bonus, it is still stronger to farm wet forests and to mine forested hills. Maybe after you have enough farms, one can leave several forests in place?
A 25% speed buff for worker actions is nice, but not game changing. The woodman's promotion on melee units (upgraded Mohawks) could be a more significant factor, I think, but even that is pretty weak.
I don’t this is a fair characterization at all. I have played them in forests (but not boreal maps per se) a few times, and I could not do anything special or unique with them, and I was trying. I think forested starts are strong for most civs though. Early chops are awesome.
Firstly, the Iroquois can get bonuses on their cities faster on forested maps than any other civ except maybe a lucky Incan start. The reason? Their workers don't end their turn in a forest tile. It IS game-changing in this respect as 10 forest tiles can mean the Iroquois save up to 10 turns of walking before they can improve on the next one. These turns saved can have an impact on your cities, and even your whole empire. I've made a couple of test runs with this before and I actually managed to get the Oxford University and, because of the latter's free tech that allowed me to buy a Renaissance tech, Rationalism faster with the Iroquois UA than without.
Additionally, the fact that they don't need roads for city connections mean they can get save a lot of gold from road maintenance. Connecting a city six tiles away from your capital with forests in-between mean 6 gold per turn saved. When that 6 gold adds up over time, it could mean you are able to buy a new unit or building, you have more gold to counter unit/building maintenance, you can buy city-state influence, etc. Do mind that you will
still want to build (rail)roads for certain reasons, like the railroad's production bonus, but until that point, you are getting a lot of gold from just city connections.
Finally, with the Longhouse, forests for the Iroquois don't give you better food or better production per se, but they do give a good middle ground, being able to produce better than food-oriented cities or grow better than hill production-based ones. With ten forest tiles, an Iroquois city can support 5 more citizens (10 specialists with Freedom) that can be overall more productive (not just in hammers but in science, gold and culture) than a mining city. That, or it can build units and buildings faster than a lumber mill or farming city. Even the Incans cannot outproduce them with their normal workshop if they convert their hills into terrace farms. It's either that (being the middle ground), or you could even build trading posts or forts on your forest tiles. The Longhouse makes these tiles incredibly versatile.
The fact, however, is that they cannot do this without forest tiles, which is why they're low tier: they're too dependent on forest tiles to do anything, and the map generation except for Boreal and Arboreal maps isn't always covered with forest tiles. With them, on the other hand, they're actually very strong and should not be trifled with.
Edit: Additional note. Arguably, the most important thing about being able to move freely in forests is that you can also defend more easily in forests. When coupling the UA with the Mohawk Warriors, it makes things even more difficult for invaders to take an Iroquois city down. A forest tile already hampers vision, movement and range for non-artillery ranged units. In fact, ranged units are practically useless when every tile is a forest, unless they can manage to secure hill tiles. In the end, you cannot simply just try and take a well-placed Iroquois city to try and take them down, especially before Artillery is researched.