The cheapest bikes have 10mm axles with nuts on. They obviously have 10mm wide drop-outs to fit the axle, so the 10mm motor axle at least goes in.
More expensive bikes have 9mm QR axles and drop-outs to suit, so they nearly always need filing.
Look at the thickness of those drop-outs. if I wanted to convert a medium priced bike with a front motor, that would be a good candidate, though I'd always fit a torque arm on the brake side tied to the brake caliper mount.
Everything considered, though, the kit looks pretty good. I'd prefer the Swytch way of putting the controller in the fixed mounting rather than in the removable battery.
I'm doing an ebike conversion project with a 15 year old to improve his job-seeking chances. We're looking at something similar to that kit, but not with the battery sticking out in front of the handlebars like that. He's writing it all up in a big folder with all the details of his learning experience about how it all works, the construction techniques, design choices, safety of operation and construction, self-designed parts, constraints and regulations. The guy is dead keen, and never done anything like that and never made anything before. I set him homework each week on aspects to study, and he does it all very thoroughly. I'm also tutoring him in Physics, which is where it all started from. He was very lucky coming to my house to get one to one tuition, a free electric bike conversion and prospects of a very bright future in technology. How many 15 YOs can tell you about how to make a lithium battery, what sort of chemistry is best , how to solder and weld, what a BMS is and how it works, how to build a wheel, how an ebike control sytem works, the role of microprocessors and sensors in a control system, and things like that? His first homework was to research the regulations, second was to study the diferent types of ebike kits and figure out what the main components are.