Part of the problem is that they really were V8’s with two cylinders lopped off. Without the additional engineering needed to make a 90 degree V6 work properly, they didn’t do well.
You’re right - the early Ford and GM V-6 engines were not the smoothest. As Audi has demonstrated, a 90 degree V-6 can be made to be acceptable but it still requires tweaking the spacing of the crank throws, balance weights, counterbalance shafts, etc.. which all add extra complexity. The Germans naturally take this as a challenge and are never afraid of building something complicated and/or expensive. LOL!