Simply put, TLDR: all religions should be somehow viable, a good choice one way or another, each with their unique bonuses and mechanics as an appeal; it should be a strategic/tactical choice in which religion you want to convert to, not because it's simply a no-brainer to convert out of a deadweight.
Having different mechanics is fine, like:
Muslim with Piety, u want to keep it at one side
Buddhist with Karma, u want to balance it
Orthodox with PA, max it
Hindu with Deity, take your pick...etc
However when it comes to conversion strength, stability, or tolerance, each religion should be at least good at something:
like Christian - conversion all the way (esp Coptic/Orthodox) - No Unrest decisions
Muslim - conversion (used to be) and also good at tolerance with Dhimmi - Ruler stat determined conversion/unrest decision
Hindu - tolerance all the way, and with a unrest decision
Tengri - great tolerance with some not-bad generic pagan unrest/stab cost decision
Then we got the rest, which are good at NOTHING, like Buddhist - No bonus tolerance of true faith, terrible conversion strength, minimal missionary, there used to be +1/2 tolerance to heathens when i did TTM in 1.14 and now those are replaced by useless crap, so you might have better luck with Hindu. (no brainer conversion)
The list goes on, and ironically, Tengri & Fetishist might be better choice in a lot of cases (even though the game kept asking you to convert away from pagan)
Having different mechanics is fine, like:
Muslim with Piety, u want to keep it at one side
Buddhist with Karma, u want to balance it
Orthodox with PA, max it
Hindu with Deity, take your pick...etc
However when it comes to conversion strength, stability, or tolerance, each religion should be at least good at something:
like Christian - conversion all the way (esp Coptic/Orthodox) - No Unrest decisions
Muslim - conversion (used to be) and also good at tolerance with Dhimmi - Ruler stat determined conversion/unrest decision
Hindu - tolerance all the way, and with a unrest decision
Tengri - great tolerance with some not-bad generic pagan unrest/stab cost decision
Then we got the rest, which are good at NOTHING, like Buddhist - No bonus tolerance of true faith, terrible conversion strength, minimal missionary, there used to be +1/2 tolerance to heathens when i did TTM in 1.14 and now those are replaced by useless crap, so you might have better luck with Hindu. (no brainer conversion)
The list goes on, and ironically, Tengri & Fetishist might be better choice in a lot of cases (even though the game kept asking you to convert away from pagan)