Searched multiple sites but could not find the answer to the question if it is legal and safe to have an electrical breaker panel installed outside of an external wall of a single family home in Ontario Canada.
There are plenty of panels which are UL listed for outdoor use; that's what NEMA 3R is.
However, the outdoor environment is tougher on panel components. For instance GFCI and AFCI breakers will have a much shorter service life outside. The threats are dust and moisture (mostly via condensation).
It has been suggested to me that placing a rather small heater inside a panel can help drive off condensation, but of course will cost a small amount of money (typically $1 per watt per year) and will alarm anyone who looks at your panel with an infrared camera (normally warm panels are a BAD thing).
One option is to put the main breaker outside and the rest of the service panel(s) inside.
If this is part of a recent trend to eradicate all utility spaces from a home, I think that trend will backfire. Houses have utilities (unless you're Amish). Utilities need maintenance.
As others have stated, if it is rated for the location and work done to the Ontario electrical code standard then it is okay. Farms in Ontario might have their main service mounted on a hydro pole( meter and main panel). From there it branches to sub panels in the house and in the barn. You didn't include pictures or specifics about location so further details might sway my opinion.