Freighters often do much shorter trips with fuel stops as they are heavily loaded.
Freighters likely seldom go out with full tanks as they are most-effective when loaded to near MZFW, which limits how much remaining of the MTOW can be used for fuel tankage. The 747-400ER and 747-8 could do Asia to North America non-stop, but they would trade many multiples of payload weight compared to fuel weight to do it. By doing a stop in ANC, they can take their full payload and burn little to no extra fuel because the trip fuel for the two stages (say PVG-ANC and ANC-PVG) is lower thanks to a much lower take-off weight compared to doing PVG-MEM non-stop and having to tank the fuel for the entire journey.
FedEx is an exception with their 777 non-stops between China and their US hubs, but they carry mostly light packages (consumer electronics like phones, tablets and laptops) so they don't come near MZFW (instead hitting their volumetric limits) so they can tank the fuel necessary for a non-stop.
I would think an A330neoF does have a clear niche it can dominate. It's much more economical than a 767F, and much smaller than a 777F. You'd think that combination should generate some sales.
The A330-200F is pretty much spot-on as an MD-10F replacement in terms of payload volume, payload weight and design range. However, it doesn't fit in an MD-10 ramp space (it needs the next size up). The 767-300F will carry 90% of the payload volume of an MD-10 and 75% of the payload weight 10% farther and will fit in an MD-10 ramp. Hence why FedEx and UPS, operators of large A300 (and A310 in the case of FedEx) went with the 767F over the A330F when it came time to replace their MD-10s. The 767F will also carry more payload by volume and weight farther than an A300F or A310F so it will almost assuredly be the replacement for those fleets, as well (and in fact the 767F has replaced the A310F at FedEx).
But for operators who have larger ramp space, the A330-200F has often been the platform of choice. Operators like Qatar and Turkish, who also operate the 777F and 747F, have chosen the A330F over the 767F.