I have a full sized backhoe that I use for tree removal all the time. I've taken out thousands of trees with it. There isn't a tree that is too big for it to take out. I can't speak for the small excavators, but would think the process is the same.
Each species of tree has a different root system. The bigger the tree is, the bigger the roots are. Cutting through the surrounding, outgoing roots is the biggest challenge. I've had some that where so thick that I had to start digging 20 feet away from the tree to get through the roots. That is an extreme case, but ten feet out is fairly common on the bigger trees.
The most dangerous encounters that I've had are with saplings, when pulling them back and messing up so that the tree or a branch hits me as it goes down. That is operator error, and in every case, it's been my own fault. Those little trees bend and twist in all sorts of directions, and when taking out dozens of them or more in one spot, I sometimes get ahead of myself.
The scariest thing to do is take down a standing dead tree. Once you start digging around them, branches or even the top of the tree can snap off and come down on top of you. I had one close call when the top part of a pine came down off to the side of me. If it had been over a little more, I might be dead. You never know with these things, but it was a good lesson that I learned the first time.
NEVER cut the tree before taking it out. The height and weight of the tree is what will take out the root ball for you. Even a leaning tree is perfectly balanced by it's root ball. Dig a ditch all the way around it and you will find that it still takes some effort to push it over, or find the sweet spot where it will go over easily.
Use the corners of the bucket to dig under the roots and under the tree as much as you can. Especially on the side you want it to fall. I can put a tree down within a foot of where I want it to go. Of course, you have to pay attention to the tree, and plan ahead how you want to dig it, where you will put the dirt so you can get to it from every angle.
Getting the tree down is pretty quick. I would think that if you get the hang of the controls and what it takes to take downa tree that you can get all 30 down in a day pretty easily. Just don't cut them first and try to dig out the stumps. If you are digging stumps, it might take an hour or a couple of hours to get out each stump.
What you do with the trees after they are on the ground is where you will find the real work begins. I drag or carry them to a burn pile. Nobody wants to buy them in my area, and for me, burning them is the fastest, easiest, cheapest way to get rid of them. I have a grapple on my backhoe that will pick up quite a bit, but for the bigger ones, I drag them with a chain. Some of the big oaks have to be cut up into managable pieces, and that really eats up the hours.
Start small and work your way up to the bigger ones.
Fill in the hole as soon as you can. If it rains and fills up with water, it will take years to dry out and the soil in there will turn to quick sand. It will feel solid to walk on, but it will swallow up a tractor when you drive over that spot!!!! I've learned that lesson the hard way. LOL
Good luck,
Eddie