I haven't used ProBikeKit.
I have ordered from a number of overseas companies: Ribble, Wiggle, and Merlin, and Bike-Discount.de (use for Wippermann, Bausch and Lomb, and German products), and haven't had any issues with any of them.
Over time they've made changes, especially with respect to selling Shimano components to US customers, as well as changes in pricing for shipping. Just watch your shipping costs.
I suppose the one annoyance is that stock can come and go, and they are oriented towards selling common items. Stocking issues could be amplified by COVID.
I haven't ordered from the European vendors since COVID, but expect an extra delay in shipping.