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Yes. writing word in kanji or katakana sometimes works as emphasis. In other words, writing word in hiragana works as not to be emphasised.

Writing word in katakana that usually written in kanji is also works as emphasis with a bit different meaning. It tend to be used as stereotype. For example, 福島(Fukushima) / 広島(Hiroshima) is just a name of prefecture, but sometimes written フクシマ / ヒロシマ that refers nuclear plant accident / nuke bomb event. (I really dislike this usage).

Sometimes all-katakana is used in fiction to indicate foreigner or robotic voices (like the Starmen in Mother 2). Writing a scream as a string of ア's instead of あ's gives it a piercing quality, moreso when you add the dakuten marker (゛), even though it doesn't change the pronunciation in this case.

More of an indication that implications exist, e.g. ガンバる implies it’s supposed to be but not in kanji, ヒロシマ or フクシマ implies nuclear context.

They can be used to convey tones in text similarly to how italics, caps, symbols and other decorations work in general, I think that’s what they meant to say by emphasis.