I've built many guns from Pecatonica River. None from TOTW.
Difficulty? Well, there's two kinds of "kits".
The kind from the factory which are basically factory guns pulled off of the assembly line before the metal is finished and before the stock is given it's final sanding and finishing. The barrel and other steel parts can be browned, blued or polished and left "in the white".
Because your working with a semi-finished stock, you can add inlays, patchboxes or other special things to make your gun a step above anything you could buy from the factory.
These are great for first time projects, although I've seen some of them messed up by oversanding or leaving the mill marks on the barrel before giving it the final finish.
The "kits" from TOTW, Pecatonic River, Chambers and a number of others are basically a box of rough parts. They need final fileing, fitting, drilling, trimming, sanding, forming, assembly and finishing.
You can pay them to do some of the machine work like fitting the breech plug, cutting the dovetails for the sights, installing the butt plate, the barrel underlugs and a few more things I've forgotten at the moment.
The mortice for the lock and the trigger guard are semi-finished but require you to do the final fitting.
The trigger guard and butt plate are rough sand castings which require filing, sanding and polishing.
Things like the lockplate and the trigger assembly which require screws to hold them in place will have to be drilled and tapped for the screws.
The underlug pins will have to be drilled and installed by you after the barrel is in place.
All of the small parts like trigger guards, side plates, thimbels, nosecap, toeplate, patchbox and any special inlays you might want to add will require you to do the work and fitting.
The barrel will require draw filing and some sanding (except Rice barrels). They will then need to be browned, blued or left in the white.
The list could go on, but this gives some of the highlights. It's a lot of work. Fun work, if you have the patients and are somewhat handy with tools.
There are a few companys which offer their guns "in the white" meaning the gun is basically finished but you have to do the final sanding and finishing. These are the closest thing to a "factory kit" available which produces a non-factory semi custom gun.
There is much to learn about building a rifle (or pistol) from any of these kits but there are some good books out there, and more importantly, there is this web site.
Builders Bench was created to help folks build their guns and a lot of very good builders are here to answer even the smallest questions.
A word of warning about any of these kits. Once you've built any of them, you will be bitten by the bug.
Before even finishing one you will be thinking about the next one. ::