I think if the motor could keep up the bike would be fine.

You are not quite correct. The more powerful a bike's motor is, the beefier the frame and wheels have to be, or it will literally bend or break when you apply too much torque. You also need a strong suspension, thicker tires, and (most important) solid disc brakes. So most bikes that are very fast and powerful end up looking more like small motorcycles.

There are some relatively "stealthy" ebikes that go as fast as you want, however:

• https://bakcou.com/products/mule-fat-tire-electric-bike

• https://hpcbikes.com/products/recon-m-1?ref=dc

But they ain't cheap. The beefy parts and frame add more money to the final cost than the motor, I would guess.

You could also carry a second battery if you want to double your range (assuming you get one with an easily removable battery).

The biggest is probably just efficiency, both with aerodynamics and how motors work (from a quick search, rolling resistance seems to go up with speed, too). Faster is more work thus less range. That and battery (or just total) weight.

Personally I went with the cheapest, smallest, and lightest ebike I could find that still has gears. Aside from trails, it's easier to lift/store and transport for 1-way trips. I definitely couldn't go 70 miles, 35 miles is a maybe (I have gone ~22 miles before as someone relatively not in shape, also battery indicator on my bike seems to be unhelpfully cautious/lacking resolution).

I feel like paying 2x+ what I paid isn't worth it either (especially when they have worse aspects) but cheap is probably not for everybody, at very least because it's a 250w motor so it doesn't go very fast (particularly on its own, but gears help). Haven't ridden it in months because my local trail won't be finished with maintenance until next year (I have no navigation and I don't want to deal with pulling into ditches).