I don't think it is a matter of being OP insomuch as it is a matter that trees themselves are OP.
In most of my playthroughs, I don't even set up a tree farm for the longest time. I just invest in an Axe of the Stream or a Lumber Axe, clear out a forest after I've automated something else, then replant that forest and wait for it to grow again later. I end up with so much charcoal that I can power most of my devices for weeks. By that time, I've probably converted over to some other kind of power, and I won't need charcoal anymore.
I wonder how much of a power issue this would be if all of the tech mods wouldn't accept charcoal or wood items for power. Trees are essentially free, and you can save on their one and only cost (time) if you use bonemeal, which all of us have undoubtedly gathered chests full while playing. Sure, you can adjust the costs of MFR planter and harvester, then adjust steve's carts, then adjust thaumcraft 4, and make it all about equal to how many materials and how much power is needed for forestry, but that will never stop some lone woodsman from setting up a few automatic machines and tearing down a whole forest by hand with a good axe or turtle.
In the end, one of two things has to happen to make it so trees themselves won't be a problem:
#1: Make it so the basic structures of all mods can't be powered by wood, charcoal, or saplings.
#2: Make it so trees themselves are expensive. Make growing trees require specific fertile land, a large amount of time that can't be fixed with bonemeal, or make a tree convert all nearby dirt/fertile land blocks to gravel each time it is grown (wherein a new process would have to turn gravel back into dirt, then into fertile land). Something along those lines.
There are a lot of things trees can be used for, like for decoration or style, specific devices, unusual resources, but using trees for power in minecraft doesn't seem like a good idea.