When you combine some text formatting it doesn't always work:

this should be bold inside italic
+this should be bold inside underline+
-this should be bold inside deleted-
@this should be bold inside inline code@

this should be italic inside bold (works)
+this should be italic inside underline+
-this should be italic inside deleted- (works)
@this should be italic inside inline code@

+this should be underlined inside bold+ (works)
+this should be underlined inside italics+ (works)
-+this should be underlined inside deleted+- (works)
@-this should be underline inside inline code-@

-this should be deleted inside bold-
-this should be deleted inside italic-
+-this should be deleted inside underline-+
@-this should be deleted inside inline code-@

@this should be inline code inside bold@ (works)
@this should be inline code inside italic@
+@this should be inline code inside underline@+ (works)
-@this should be inline code inside deleted@- (works)

I don't think anything needs to be changed for "inline code" because of its purpose, but it would be nice if the others could be freely combined.