The Thompson lineage is reasonably correct. To say that the other small walking foot machines that look like the Sailrite are clones of the Sailrite isn't correct. More correct to say that they share a common ancestor.
There aren't a lot of difference between the Sailrite and the "clones". Sailrite has identified which parts tend to fail, and they spec those parts a little heavier. They all come from China.
All these machines are crude, including the Sailrite. Crude castings for the body, and for the internal parts. Parts that you'd expect to be machined and polished are left looking rough.
If you get a "clone" you're rolling the dice. Open the box, oil it, and it'll probably be fine, but if it's not, you'd better have an understanding of how to improve/fix it. You're on your own.
The Sailrite machines aren't a whole lot better than the others. They've made some of the parts more robust, and they have some reasonably thoughtful improvements like the stictch length control and their Monster Wheel, and most notably, their customer service is outstanding. If you have a machine that's not running right, you will get it resolved.
The Sailrite comes with a double pulley system to reduce the gearing between the motor and the balance wheel (ditto for the clones). One of the problems with the Sailrite (which may have been corrected...I don't know), is that when you get your Sailrite, the wheels in the double pulley arrangement are poorly aligned and benefit greatly from being shimmed and tweaked for to get the belts running true. Other than that, the Sailrites are typically ready to run straight out of the box.
If someone asks me which machine they should get, and they have no experience with sewing machines (using or fixing), my answer is going to be Sailrite every time. It's going to work, and if they can't complete their project, it's not the machine's fault.
If you know basic maintenance on sewing machines, and understand how they work, there's nothing wrong with a clone other than some of the parts have not been made more robust per Sailrite. But, you're probably not going to break them. If it runs right out of the box, great. Otherwise....