All good questions, which I’ll gladly answer from my experience (plus you can search topics on this forum which is very handy).
They are raw plastic. Putting some wax on them greatly helps bees get started, and it only takes a few smears of burr comb you may have a collection of to work.
Probably depends on the colony, because I’ve had it happen on occasion but not always.
It can become a very gummy problem when the Fsuper is left in the hive too long into fall in cold climates. Flow was developed in an area of Australia where it is possible to continuously harvest honey, because there’s no cold winter when forage is dead and the bees have to cluster for months to survive. Northern US-based early adopters of the Flow system have learned various important tweaks necessary for successful operation - a big one is to remove the Fsuper and queen excluder once your nectar flow is finished for the season. However, that is general advice that needs to be filtered once more through your local beekeeping community’s experience with the specific climate you live in. So be sure to check with local beeks and ask if they keep regular supers on through winter months. If so, you might be ok to leave the Fsuper on as well.
@Eva has already answered your questions, but as I was typing at the same time as her, here are my contributions:
The frames are uncoated food grade plastic. Flow is unable to wax them because of international shipping regulations prohibiting or complicating the movement of biological products. It is very easy to smear some burr comb on them from an inspection though, and my bees readily took to the frames when I did this.
There is always a little burr comb, especially at the outer edges, but not enough to be a problem
Propolis can be a problem, especially if you leave the Flow super on over winter. For this reason, I take the super off in late summer, after the end of the nectar flow. If you do get problematic propolis, it can be removed by freezing the frames, then repeatedly opening and closing the mechanism. Freezing makes the propolis brittle, and it will fall off. Other people have used a jet/pressure washer, but you have to be careful not to get too close to the frame with the tip of the wand, as the plastic can be damaged.