Certified aircraft equipped with ferry tanks usually lose the normal category and have to fly under experimental while modified for range. Overflight permits in many countries require proof of insurance which can be very hard to get for experimental aircraft.
I set up a C210 with extended tanks. I had a choice of a weight limitation and FAA 337 approval of the internal cabin tank retaining the normal certificate, or an overgross permit and experimental category. I opted for the normal category with the weight limitation, leaving some unused tank capacity. Still, the worldwide geographic coverage was very difficult to obtain, taking almost two years of searching.
The 210 has a broad cg range and with full fuel in the tanked configuration, this plane has a range of well over 3000 nm. With extreme range, prepositioning of fuel isn't necessary, but flights are rather long with attending issues of night flight and difficulty obtaining current weather conditions, etc. Careful planning certainly smooths the way, but the flights are more than just gps and autopilot.