Not if you go about it the smart and responsible way. I have a fairly customized setup, and yet here I am, running Plasma 6, with my old window decorations, my old Plasma styles — several of them — and my old kvantum theme. 
But of course, if people just open up the pamac-gui, run the update process, and cannot even be bothered looking at the Stable Updates threads, then yes, there will be problems.
Look, Plasma 6 isn’t perfect, and yes, there are several bugs — I’ve discovered a whole slew of new (but small) 15-minute bugs, even — but if you consider how much work has gone into getting both Plasma 6 and the KDE Frameworks 6 out by a deadline — which in and of itself is not such a good strategy — then it’s amazing how stable and usable Plasma 6 is.
Granted, I’m still on X11 because there are still some obvious issues with qt6 on Wayland, but if you update the smart and responsible way, and you read up on the caveats on the Stable Updates thread, then there’s very little to worry about.
Back when we upgraded from Plasma 5.24 LTS to 5.25, I had far greater problems, and Plasma 5.25 was completely unusable for myself and for many others — about half of the Plasma users on this forum. At my own responsibility, I then kept my system in a partial-upgrade state by skipping all Plasma-related updates until 5.27 LTS was released, and several other people here did the same. But compared to the transition from 5.24 LTS unto the for many people broken 5.25, the upgrade from 5.27.11 to 6.0.4 now was a breeze — pun not intended.
Again, there are bugs, but they are all fairly small and mostly cosmetic ones. Everyone else who’s had problems with the upgrade from 5.27.11 to 6.0.4 — I’m not going to be addressing GNOME or Xfce, because I don’t use either of those — owes those problems to not having done their homework and not updating responsibly.
There’s a reason why I wrote this little HowTo…
But nooo, people are allergic to the command line. They don’t want to update from a tty. They want point & click. Well, guess what, they’re the ones with all the issues now, and as usual, we see dozens of threads popping up that are all about the same issues, because nobody bothers to even check whether their issue has already been reported, let alone that the solution and other advice was already given on the Stable Updates thread from before they updated their systems.
Look guys, if you don’t want to be responsible, then that’s all fine. But then don’t come crying that Manjaro broke your system. That’s what the word responsibility is all about. We can put the food on the table in front of you, along with a spoon and a fork. But if you’re going to be spitting it out, then you’re going to stay hungry, and then that’s not our fault.
Switch to the Breeze window decorations, application style and Plasma style before the update. Then log out, reboot, and instead of logging in at the GUI, log in at a tty, delete the contents of your ~/.cache/, stop sddm — you should definitely know how to do that by now on a systemd-based system — and run the update. Then reboot again, log into Plasma and restore your theme settings.
Don’t believe everything you read on the internet. 