I have a closed system but I now have the option to play with various materials. One thing that I give attention to is to have PLA nozzles and ABS/PETG nozzles. Only because I know what carbon residue can do from burned -anything-. The more it "carbonizes", the more stuck it gets. It is a molecular bond thing highly dependent on materials, of course, but I do know that PLA will turn to carbon over time with elevated temperatures. My machines run ABS at 260C and first layer PETG at 265C. If that nozzle has any PLA remaining, it will soon become a block or a dam within the tiny space of the nozzle aperture. Dust particles can do this too but normally they are just swallowed up in -melt- and become black blobs, also problematic over time. Only recently did I learn about "cold pulls". If there is any hope of removing stubborn particles from inside your nozzles, that would be it. Unfortunately, my nozzles aren't quite as simple to maintain as most.

I think that melt and extrude issues are the hardest things to get over when you need to dial in a material or a special print. I prefer the closed systems for that alone. You know it -should- work. Good recipes, which our profiles are, are not simple to come by.