I took a trip last weekend using a few DC chargers. I pre-planned last week, and it indicated that I didn't have enough range, and gladly routed me to the Electrify America chargers on the route. When I left on Friday, I tried to use it in real time and suddenly it couldn't find those chargers! Instead, it was trying to route me to Tesla Superchargers (?!?) which the SE cannot use. So yeah, I don't think it is well implemented.
Fortunately for me, I am used to traveling with my Bolt so I already knew of all the chargers I would need for the trip. I was hoping the Mini's nav would find them for me, but no dice. So now I know not to rely on it in unfamiliar territory.