Reasons to Restrict Multiclassing?

[Tanarii]

Where is that at?

Page 285.

It most certainly does not say to do it as you want. It says to make sure the swap is just as appealing and just as effective across all three pillars. It does not, however, say to watch out for overpowering things.

SUBSTITUTING CLASS FEATURES
If one or more features of a given class don't exactly
fit the theme or tone of your campaign, you can pull them out of the class and replace them with new
ones. In doing so, you should strive to make sure
that the new options are just as appealing as the
ones you are removing, and that the substitute class features contribute to the class's effectiveness at social interaction, exploration, or combat just as well as those being replaced.
Ultimately, a class exists to help a player express a particular character concept, and any class feature you replace is also removing an aspect of that character. Substituting a class feature should be done only to fit a specific need for your campaign, or to appeal to a player trying to create a specific kind of character (perhaps one modeled after a character from a novel, TV series, comic book, or movie).