Three years after they were announced, it appears that Thunderbolt 3 optical cables are becoming more affordable, especially now that Corning has started selling its cables in lengths between five meters and fifty meters.
Although Thunderbolt 3 optical cables allow for longer lengths, they have some limitations compared to traditional copper cables, the most important being that they cannot power a device. This means that small accessories such as external hard drives that need to be powered by the main device cannot operate using optical cables.
Thunderbolt 3 optical cables are only compatible with Thunderbolt 3c connections and can't be used for other slower versions of USB, so you can't connect USB-C devices that aren't Thunderbolt 3 compatible, for example.
However, they can be used with Thunderbolt 3 docks that can offer other connections like USB, display, audio, and more, as the Thunderbolt controller in the dock does a conversion of everything that works on Thunderbolt. Various Thunderbolt 3 accessories can also be connected consecutively (daisy-chained) using optical cables.
As you might expect, all of this optical technology comes at a price. The shorter Thunderbolt 3 optical cable than those sold by Corning, which measures five meters, costs 474 euros (bought in France, in Spain they don't have a distributor yet). However, the longest cable, 50 meters, costs 618 euros, a small increase compared to the increase in cable length.