The reason I would not do these is different than what many folks have stated here...
There is something sacred about a person's religious beliefs, whether the person be Hindu, Buddhist, Islamic, Jewish,.Christian, Shinto, Norse, etc... I willingly will visit places of worship for any faith if I am there as an attendee for a ceremony, to support a friend, or to learn more about a particular religious faith or practice. Heck, when I am lucky enough to travel, I love visiting local places of worship and learning about how folks practice their particular faiths - it's a great way to learn about a culture. However, to go to a worship service as an evaluator feels sacrilegious to me - even if I want no part of the faith itself.
I get it: The church leadership teams want these evaluations. Attendance in Christian churches has been declining over the past several decades and their aging congregations desperately want to make their services to be warm and welcoming. I have no issues with this pursuit. I simply cannot go to a church under false premise.
Hard work builds character and homework is good for your soul.