The Mamluks should definitely send their entire army to defend the Touggourti people from the horrible Mozabite Ibadi heretics instead of, you know, defending themselves from the Ottoman onslaught sieging Cairo. Makes sense.
Or alternatively we have the legendary Transoxianan army marching through the entirety of Iran and crossing the Hindu Kush to defend Nagaur against the Hindu heathens of Mewar.
You sometimes even get the laughable situation of the Timurids 'defending' Brunei against Kutai, without a navy. Have fun sitting on a full occupied Brunei for 5 years.
The new defender of faith mechanics have absolutely ruined smaller countries that are not Catholic or Sunni (Serbia, Mzab, Oman, Ryazan, etc.), because the AI will instantly re-buy DoF as soon as they lose it, or if they don't one of the several other great powers will. Does barely white peacing France out as Serbia after a grueling war, only to have Spain going 'lol 500 ducats go brrr' and taking DoF instantly sound like a fun experience to you? Or how about bankrupting yourself fighting both Tunis and the Mamluks as Mzab only to realise that fucking Kilwa has somehow taken DoF.
And of course, due to how AI military access works, you won't actually be able to reach whoever's the defender of faith, while they'll send doomstacks that carpet siege your puny countryside.