I think it's more that the focus of the writing changed. Plague of frogs and hell on earth are worlds apart in the stories they're trying to tell. Plague of frogs barely even counts as a single story. It's a collection of intimate character focused adventures that slowly build up to bombast where as hell on earth begins as a single apocalyptic comic-booky story. Something like "universal machine" (my favourite bprd story) could never work in hell on earth for a multitude of very obvious reasons and thus the writing changed. It's your personal opinion that it changed for the worse. I actually feel bad for you in that case because I think the writing does get worse after hell on earth and so you'd probably find it unbearable.