^^^Fungi is technical jargon. And it may be scary to hear but I have indeed heard cactuses. Criteria is indeed the common plural, but does it count as a genuine exception? Or is it technical jargon too?
In any event, the -s/-es plural rule has exceptions, as all children know.
I'm not sure when it was finally decided, but no Latin or Greek grammatical rules are now held to apply to English. Also, common usage is privileged.