Setting aside fairness, how would the government work out whether I am gone or not? Lots of people go away for several months. Policing the system becomes impossible, which is why I described vancouver's system as voluntary.

Say you own three properties in Vancouver, best case you can only occupy one at a time since you are not a quantum entity from another dimension. The city could ask for proof of a tenant renting the unit, and if you fail to prove it, you then need to pay empty unit tax on 2/3 of your properties.

Someone being gone for a stint and leaving a single apartment unoccupied for a while might not be a big issue in the grand scheme of things, if most of the problem is from people who can afford several properties. If the city wanted to go after these sort of units, maybe they could pull utility data and see a drop off in use. I'm no plumber, but with Vancouver winters, maybe you do want to have your water service shut off if you will be gone long term? No one likes coming home to a house completely encased in ice inside and out.