I came to FreeBSD/PC-BSD because of ZFS. And even though ZFS support in Linux has improved, it still sucks in various ways (e.g.
zfs allow doesn't actually work, forcing you to do everything as root, and aside from Ubuntu, no linux distro puts the ZFS drivers in the kernel, making root on zfs incredibly risky). So, I would hate to have to go back to Linux. That being said, KDE is my DE, and my normal workflow relies on the functionality and programs that it provides (e.g. you're pretty much going to have to pry konqueror from my cold dead fingers). So, if KDE stops working in FreeBSD/TrueOS, I will be forced to switch back to Linux and deal with the pain that comes with that - and that will not be fun.
The fact that KDE 5 is not supported by FreeBSD yet is problematic from a support standpoint, but KDE 5 also lost some functionality in comparison to KDE 4 (at least for some of what I do), so I'm not all that heartbroken about using KDE 4 for now as long as it works.
Regardless, while I think that it's great that something like Lumina exists and am sure that it will work quite well for many folks, I'm just too invested in KDE and its ecosystem to switch away from it unless I have no other reasonable choice.
Honestly, from where I sit, no OS is really a good solution. It's question of tradeoffs and which sets of pain you want to deal with right now. I much prefer FreeBSD/TrueOS' approach to the OS, but Linux is much better supported in that many projects target Linux (with some working on FreeBSD but many not), whereas relatively few target FreeBSD. So, I'm forced to have a Linux box to do certain things just like I'm forced to have a Windows box to do certain things. None of the OSes do everything that I'm looking to do, and it sucks.
Right now, I'm able to use TrueOS as my primary OS in spite of the problems that I have with it, but while I think that it's great that we have some projects targeting FreeBSD, what I really want to see more of is programs just working everywhere. And unfortunately, it seems like a number of the choices being made in the Linux ecosystem really don't jive well with that at all.