Cable cars are a poor and misleading analogy for interoperability -- that is truly a difficult, one of a kind system, with significant mechanical and switching limitations that don't apply to streetcar systems.

For streetcar systems, by contrast, most cities used the standard PCC car from the mid-30s onward, with minor variations. https://en.m.wikipedia.org/wiki/PCC_streetcar

As for dangerous? During the multi-decade period in question, your link lists two accidents in the US. One is the crash of a gasoline tanker that also destroyed five buildings, which I don't think we can lay at the feet of the streetcar itself.