Christianity

[Rahwana]

Con
-No warrior monks.
-Need to convert conquested provinces can slow down an advance.

warrior monks can be killed easily by spamming guns
just get a big sized garrison

and Shimazu is the easiest

[Monk]

warrior monks can be killed easily by spamming guns

And if that doesn't work.. use more gun.

[nafod]

In case anyone was wondering I really don't see a reason not to convert unless you're playing as Uesugi.