I respect your experience & you must have ridden more powerful bikes than what I have. Mine experience is with 100-150cc bikes both Honda.
But on my Test-Ride of Street 750 the Brakes indeed made me nervous.
What I have been reading from various sources is compared to likes of Honda, Harley's have always performed below par. May be threshold is different if you have ridden Hondas all your life.
MoreOver Brakes on Sportster Range of Harley has been Uprated from year 2014 only.
There is no way on earth the Street 500 or Street 750 will compare with a 100-150cc motorcycle when it comes to either braking or handling. For someone stepping up from a small motorcycle I can see how it would make them nervous both related to braking and handling. They just aren't the same.
I would remind everyone that a 500cc and 750cc motorcycle are not small motorcycles. They used to be considered large motorcycles because they are large motorcycles. The fact that we have huge and heavy cruisers notwithstanding those are the "tanks of the road" and we could compare them to an RV and a car. Touring motorcycles are the RV's of the motorcycle world.
Having read reviews of cars and motorcycle for years I've learned that these are expert drivers/riders that often compare vehicles based upon racing performance standards. For example my partner rode my 750 and said it "pushed" in a turn. His opinion was based upon yeas of experience in professional motorcycle road racing and, while valid, it compared a street motorcycle with a professional road racing motorcycle.
I read, for example, in Car and Driver that the 2012 Fiat 500 was "under-powered" and I purchased one anyway. My stock Fiat 500 easily reaches over 60 mph on a freeway onramp and has more than enough power for any normal street application.
My Sportster is a 2007 XL50 and has zero braking problems and I've found zero braking problems with my Street 750.
If you want braking problems you should have driven my 1928 Oldsmobile with mechanical brakes. Press the brakes and pray was what stopping was like. You actually had to use the emergency brake in many normal braking situations. LOL