I see that TC 10.x doesn't include the the NUnit3 runner, why is that? The NUnit task has always been easy to use, but now you need to set up a path to the agent or relative to your package folder. This is cumbersom and both will break in time. The best solution for me would be to choose "Lateset NUnit3.x version" and then TC would choose the latest version it ships with.
I have never had any problems with NUnit breaking their API within the same mayor version. Is this the reason to set the NUnit runner explicit?