The easiest way to approach hub motors is to look at the total weight (bike + rider) and the grade of the hill. Climbing is totally weight dependent so more takes more. Hub motors will bog down with some combination of weight and grade. I'm about 230, on my ebike, and my Mac will draw 1000 watts (which isn't power to the wheel). I can get up a 12% grade, but it's clear the motor is straining. The grades I have to climb are around 8% max, and the geared hub handles this very well.
Most people have a rough idea of what grades they will climb, especially on paved roads. If you want to climb hills off road, the mid-drive is hard to beat because the grades are likely to be very steep.
That said, 3000 watts is a lot of watts. I plugged 3000 watts into a cycle computer using a 20% grade, and it's moving right along. Remember, the motor may only be 70% efficient, which makes the 3000 battery watts around 2000 at the wheel. So you may need a 60 amp controller to get 2,000w of actual climbing power.
http://bikecalculator.com/how.html
This doesn't mean that a very basic motor from Ebay will do this. It's easier to ask what grade people need to climb, and what weight they will be carrying. With the legal limits, basically 750 watts, a mid-drive starts to make sense as you move above 10% grades. You would want to design a bike with 3000 watts differently from a standard ebike, certainly beefed up metal for the torque loads, beefed up brakes for the heat they have to dissipate getting stopped.
You could certainly look at a more refined mid-drive if you are lucky enough to have dealer support. You may be able to tweak your drivetrain to make it more reliable. The Bafang ends up on everything under the sun, and sometimes it seems to be pretty flawless, and other times it is less reliable, just going by user reports.
