I think this review you linked nails one aspect:
"Here you have a great cast, a successful director, screenwriters who have written other successful work, a chunky budget . . . and yet something went wrong. What was it?
“Tone” is such a strange, ineffable quality in any dramatic work, but it’s also one of the most vital."
I just couldn't believe the clunky Father Brown aspect of the whole thing, and the hammy acting, and the suspension of disbelief involved in having to accept they'd be living in a multistory listed castle (without, as she pointed out, any lifts).
I was also really disappointed they didn't show almost any of the actual gatherings of the club or their discussions. Instead, it was all Helen Mirren running the show and solving things and swooshing about in her expensive car. That's what I imagined it would be - them at their crime board, solving an intricate crime in ingenious ways - not a tone-deaf farce.