You don’t mention speed or which kit. I find that high quality rim brakes and KoolStop salmon or KS ebike specific pads give me good braking up to 20-25MPH. If you are considering a hub motor regen braking can be quite powerful.
@m@Robertson i think it was didn’t like AVID BB7 mechanical discs but i found them basically ok and solid to 25mph. I grab a friend to help me field test braking. I use the MSF drill for avoidance and then panic stops to determine just what my braking distances are. You can always slow down and upgrade incrementally. I’m trialing Magura hydraulic rim brakes for front use at some point this winter.
I used a set of these on one bike with BBSHD and KoolStop salmon MTB pads. I wasn't happy at 30MPH. But it was more than just brakes that cooled me to those speeds.
With good brake pads, I have 26” discs.
From ES conversation with a fellow I've learned much from,
The various Pedersen SE brakes (Scott, Suntour, and later Tektro) were as potent as cantilever brakes ever got, and the best available before Shimano introduced V-brakes. But good linear pull brakes with the right levers and pads are just as strong as SE brakes were, and less temperamental. For better lever feel and maybe even more bite, you can add a booster arch to stiffen the pivots.
If you can't get more braking power than you can stick to the ground with something like Avid Single Digit 7, Speed Dial levers, Kool Stop pads, and a booster, then there's a problem with the cable or adjustment. (Or you're running a heavy multi-passenger pedicab.) You can turn up the gain with long blade levers if grip strength is a limiting factor.