Most DIY kits that are added to a standard bicycle frame are typically "hackable", and also very simple in construction to keep their price low. When you look at countries around the world, there are a wide variety of power and speed limits, and the companies that make ebikes are trying to navigate all of these issues while trying to be more successful than the competition.
The vast majority of citizens who have just recently realized that ebikes exist, do not want to risk getting a citation from the police, or to have their expensive new ebike confiscated by the city government. So...this has divided the manufacturers into two camps...One camp that is occupied by the mainstream manufacturers who make built-in motors that have a proprietary mount (cannot be transferred to another frame), and are hard to hack. Many EU countries have a 16-MPH speed limit for the application of power, and a 250W power limit (Austria and Switzerland have a 500W power limit).
In this instance, if you are pedaling 30MPH and are stopped by the police? It is demonstrable that the bike cannot provide more than the factory-rated power and cannot provide that power at any speed above the legal limit. Meaning that: if you are traveling 30-MPH, it is because you were going downhill or you are a very strong bicyclist. You can still get a ticket for reckless riding, but your ebike will not be confiscated due to having 1,000W, or...because you were applying electric power at 30-MPH
Kits are different. Besides the obvious consideration that they are designed to be able to attach to a wide variety of common bicycle frames, and can easily be swapped from one frame to a completely different model. A kit manufacturer can claim that his kit is "for off-road only", but...when riding with it on the city streets, most police are still in a fuzzy state of awareness where they don't pay any attention as long as you are not riding in a way that endangers everyone around you.
Stealth ebikes in Australia started out making 3000W+ ebikes in Australia, where the legal power limit is 200W, so...up until recently, all of their products were for "off road only". (I have soldering irons that use more than 200W)
Bafang is a company that is trying to make inroads in both worlds. They make well-known kits, but...a couple years ago they also came out with their "Bafang Max" drive, with a proprietary mount (similar to Bosch and Yamaha), which can be hard-wired to meet the power/speed limits in various countries (meaning, not easy to re-program/hot-rod).
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Torque-assist pedaling is one of those features that not only cost more than a speed-sensing PAS, it is also desirable for the expensive factory turn-key ebikes with proprietary drives. Large and powerful mainstream ebike manufacturers are applying pressure to keep several existing differences between factory drives and kits. There are kits with torque-sensing PAS, but they are not common...