I'm gonna go slightly against the grain and say that it does seem like MC's various feats would cause a lot of problems. It's just that they're problems that her fiancee/the government in general should be solving.
Like if she kills the giant slug monster that'd been terrorizing an area for decades then they should be passing laws and creating initiatives to deal with the impact on the local salt markets.
Also, his reasoning for telling her not to take over the knight's jobs is dumb but it is actually a bad idea for her to be doing stuff like that, especially without permission.
For one it sounds like her job as a saint is incredibly important and she's one of the only people in the country who can do it.
For two the country needs to be able to function even if she's not around.