I have reached about 26 MPH on my e-bike and braking was totally fine. Obviously on any bike or vehicle braking distance will be relative to speed before braking, and jamming the brake is not great. But I found my bike does a smooth braking very much as I would expect and predict. And not fundamentally different than my old non-e bike did (better actually because the e-bike has nicer brakes).
To be clear, what I wrote previously was there is a max speed limit that it provides assistance to, not that it physically restrains the bike to that speed. Like any bike, you can achieve higher speeds if you sustain a long downhill run and do nothing to moderate it through braking. The result would be exactly the same whether the bike was non-powered or an e-bike with pedal assist. Other than downhill, it would be extremely hard to get to and sustain that speed eve with an e-bike which would stop helping in the 20’s. There are different certification class levels for e-bikes. Class 1-2 max assistance at 20 MPH. Class 3 at 28. Individual manufacturers can choose not to achieve those maxes but they cannot exceed them (though some bikes can be modified/hacked by users to exceed them – which is what is common in the throttle models, particularly by delivery people).