Update regarding Rhino 7: We have been using Rhino 7 for the past months and have not encountered any specific problems.
Recently, Rhino 6 has been released. The past weeks we've been working with it, and have not encountered any KUKA|prc-specific issues so far. There have been a few crashes, but they seem unrelated to KUKA|prc. However, please do let us know if you run into any issues, ideally in a reproducible way.
However, the display performance of Rhino 6 when displaying Grasshopper-planes is significantly (!) slower than in Rhino 5. Our brief tests showed that Rhino 5 is up to 80 times faster than Rhino 6.Update 06/2018: As of Rhino 6 SR5, plane display performance seems to be on par or even better than Rhino 5.
As the general display performance of Rhino 6 is better than before, this seems to be a bug. We have reported it to McNeel, where it is logged at https://mcneel.myjetbrains.com/youtrack/oauth?state=%2Fyoutrack%2Fissue%2FRH-44499
It is recommended to disable plane preview whenever possible to ensure that the viewport stays responsive.
Our testing was limited to NVidia cards, maybe someone can compare the performance on AMD graphics cards as well. The easiest way to test is to use "Populate 2D" to create 5000 points, then place the Planes through "World XY" and run the textmaxspeed command in Rhino. In Rhino 6, the viewport refreshes with less than 0.5fps on a current PC.
As long as Grasshopper in Rhino 5 and Rhino 6 are mostly compatible, we are planning to support both. Ideally, we will offer parallel Rhino 5, Rhino 6, and Dynamo builds with the same functionality. At some point after 2018 Rhino 5 support might cease, however your licenses will still be valid, and you will still be able to access older versions through the member section. Rhino 6 support will be a free update for members, as are all new builds and versions.