I'd never actually used Hidden Text, but that hadn't stopped hidden texts from appearing in my document. That's an unsolved mystery. Either way, the first point is: watch out for Hidden Text. The second point is that Outlook.com and Gmail behave differently, and I think Outlook.com's behaviour is illogical at best.
I rarely use MS Word, but occasionally a client sends me some docs, and it's easiest to review and revise in Word. Although I'm aware of the hidden text option in fonts, I never use it. I do however comment on docs frequently.
Something to be aware of, with Word.