What I think gets people more than a lack of familiarity with the role is a lack of familiarity with the champion. A lot of the time I'll see someone who's forced into a role take a champion who is good currently in that position's meta, but often it is not a champion they know how to play.
The two times I have been forced to play top, absolutely forced, I played Singed, and I played Maokai. The Singed game, I bought a philo stone and farmed under tower. Nobody ganked for either team, and my other lanes all ended up crushing. I definitely got lucky that game, but I ended up 3/0/10 on the whole because I knew the basic Singed strategy, and eventually got to the point where I could just proxy scott-free. I had only played singed once or twice before then, though, and it showed: I felt very helpless.
The Maokai game, I did similarly, but versus a Mordekaiser. I didn't know Morde's kit at all, but I knew my champion really well from jungling. Even though I did better in the Singed game, I contributed a lot more in the Maokai game because I knew my champion's limits. I helped secure objectives, I coordinated ganks with my jungler, I teleported in for teamfights. If I had been playing, say, Gnar, who I've touched maybe once in a bot game, I wouldn't have been able to pay attention to the minimap because I would have been too busy managing his kit.
