I know he had several writers involved at different times, some of which publicly stated that the working relationship ended when KS yo-yo managed. Worse, that information often made its way into books. I had a copy of the Palladium Manhunter. It was an awesome setting, lots of flavor, excellent in execution. The book was, iirc, "presented by Kevin Siembada" with others listed as the authors.
Turns out Manhunter had been published before with their own mechanics but it hadn't done so well. I don't know if it was because the mechanics blew or if it was Myrmidon Press's first book. Either way, Palladium (aka KS) had decided it was worth adding to their line.
However KS felt the need to discuss in the foreword how he took this dirty little jewel in the rough and polished into into something worth having. (Paraphrased, I've loaned the book out.) That's just not kosher IMO. If Manhunter was good enough to license, it was good enough to license. No reason to complain about how much editing and polishing was needed.
Need I say that the only errors I noticed tended to be the ones in the Palladium mechanics, where Kevin probably had the most hand?
Palladium books have horrible editing, the art varies wildly between mind numbingly gorgeous and laughable, and according to at least one former writer, the books are still published using masters rather than anything digital. The reason all those thefts hurt was that it took the litho masters.
KS has managed to keep his baby going for several decades so he's not stupid but he is somewhat egomaniacal. KS knows the right way to do things and KS doesn't do anything wrong. He's like Steve Jobs, only less successful. From what I can tell he is pretty much identical to the Gary Jackson character from KoDT.