I find early Williams ss flippers to be pretty weak. Starting around firepower they switched to a new, modern style mech, and it was stronger, but it was still 25v. Later, with fire power 2, they switched to 50v, and that stayed roughly the same until Addams family introduced fliptronic solid state flippers.

Bally and Stern used 50v flippers right from the beginning, so they were pretty strong. Stern especially. Although I find ballys linear flippers to be weaker.

In the late 80s data east introduced solid state flippers, and they were really strong. Modern Sterns are similar in design to those.

I find that a modern Stern is comparable to Williams fliptronic in terms of speed (until the flippers get hot) when both are in good shape.