At its core, it's quid-pro-quo. You do this and God does that. It's transactional.what's wrong with that?
As Jesus says in Luke 4:12, "You shall not put the Lord to the test."
In other words, don't attempt to manipulate God, which is the essence of what is happening when someone promises what God will do for you. It's far better to teach submission to God and gratefulness for salvation. In other words, humility.
Don't teach that God will reward followers with fancy cars or large homes. That's the opposite of humility. That's hubris