Columbia bought them before the SS era started. That happened around the time they went to the deeper cab with the shorter legs, and the bigger front door.
As is typical, the conglomo approach didn't serve Gottlieb well, along with other missteps they made along the way.
My view of the "built like a tank" idea and Gottlieb is, in some ways, it's true, in others, it's not. I've always felt their relays and score motors are much harder to deal with than Bally or Williams. The short throw relays are difficult to adjust. Some think that the Williams longer throw relays are harder to keep in adjustment, but I've never found that to be the cause. I've owned one Bally game for 43 years and I've had to do very few adjustments on it. I really doubt I could keep a Gottlieb running that well for that long.
I think in overall game play, during the EM era they were probably the best, although they made their share of junk, and Bally and Williams had some really good ones too. But in overall quality, I've never felt that Gottlieb had any overwhelming advantage over the other two.