Even compared to other solid state amps with the same watts, they were way underpowered.
I remember that Fogerty used Kustom amps onstage, don't know about recording. They did look cool. I was never impressed with the tone. They weren't any worse than most other solid state amps available back then, but that's not saying much.
I found this on Wikipedia---(It's mostly about Kustom PA systems, though.)
Today, Kustom amplifiers are considered fairly collectible and are preferred by some vintage enthusiasts for their solid-state tone. Rockabilly and Motown musicians originally used these amps. Other artists known for using the Kustom brand for live performances are Creedence Clearwater Revival, The Altamonts, Dusty Murphy, 3 and Sheryl Crow. Some of the most famous Kustom P.A. users include Creedence Clearwater Revival, Leon Russell, Johnny Cash, Roy Clark, The Jackson 5, Carl Perkins, Alun Tan Lan (Y Niwl), and The Carpenters. CCR toured from mid-1969 - 1972 using their own massive Kustom 400 PA system due to a lack of quality backline PA systems in venues at that time. As a result, CCR concerts were superior in sound quality, but the cost of transporting the equipment made touring a money losing deal for the band.
