I'd say Declan Finn's Pius Trilogy is another really bad one. It's not intended as a "100% undeniable" alternate history, but it has enough divergences (like say, a vastly different pope) that I feel OK including it here.
The books are written by an archtraditionalist Catholic with a gigantic axe to grind. The writing was started during the
Da Vinci Code craze, which does nothing but date them horribly. It's like criticizing 80s hair bands in a piece not released until 1999. Anyway, onto the books themselves. The first is about researchers of
Pius XII being killed and the heroes fighting to stop them. In practice, it's 1/3 clunky action novel, 1/3 the author diving into the historical controversy surrounding him (and taking his side to a
huge extent), and 1/3 author rants on stuff like how most American Catholics are too liberal and how John XXIII was a Red. And it takes itself so seriously that a scene where the pope in his super-body-armor fights off assassins who were trained from childhood in secret KGB camps is handled utterly matter-of-factly.
It only gets worse from there. Enter the North Korean-Russian-Chinese-French-Sudanese (yes, really) anti-Catholic conspiracy. In the second book,they kidnap the Pope and put him in
an excuse for more authorial rants to argue with strawman opponents a trial, in a The Hague that has mysteriously moved from The Netherlands to Belgium (this is not alternate history, it's bad research). And then of course, there's the "action", which is really clunky and researched in all the wrong ways.
The third book takes the cake, for becoming silly in the worst way possible. The conspiracy gathers an army of everything from Russian paratroopers to Sudanese machete-swingers to invade the Vatican, only to be foiled by goofy booby traps. Along with more bitterly serious author rants and infodumps. When it does change tone, it's the worst kind of tone change, where the characters in-universe don't take the threat seriously, implying the reader shouldn't either. (I didn't). The setup for the "battle" is way too ridiculous to be treated at face value, especially compared to the earlier books, but the rants are there. It just has one of the most inconsistent tones I've ever seen in any work of fiction, not even the first two books.