That's my experience as well. When I'm using a private WiFi (e.g. work/home), calls tend to be more reliable. The only times when it's also reliable on public WiFi is when I'm connected to Shaw's WiFi, and I'm also situated in one location and don't have to move around. Otherwise, my phone would constantly switch from one Access Point to another, and when that happens I've noticed some "choppiness" when I'm in a call. Public WiFi connections from restaurants or supermarkets (e.g. Supermarkets' "Supermarket Free/Guest Wi-Fi) tend to be hit or miss.
It would be pretty neat if Freedom users do indeed get priority on Shaw's WiFi over other users, that way, it would at least help alleviate some pains of WiFi calling -- especially during those times when Freedom users get little to no cellular network signal in certain locations.