The main thing is dealing with deductions for charitable giving, tuition, rent payments, etc.

This definitely ain't meant for these people (I'm one of them, it often takes me a few hours to calculate what my income is after currency conversions, and the impact of all related fx transactions). Commissioned salespeople it definitely won't work for. Nor would I think it would work for people who hold foreign real estate or significant foreign assets, or anyone that's had significant asset disposals in the past year, and probably not even people who hold second homes that are rental properties. I'm not even entirely sure about something as simple as a couple of close principal residence sales with the necessary flipping calcs that would need to take place (usually pretty straight forward, but the details can also make it be pretty complicated).

If you've got a T4 from your employer, and few deductions, then giddy up. That probably still captures quite a lot of people.