This ignores the fact that the earlier rounds are a piece of cake compared to the latter ones.
With the exception of Round 1 (easy round) and Round 6 (hard round), I find the rest rounds of every F.Brawl to be of average difficulty in general.
About TFOs, their difficulty is progressive imho:
Artist TFO (easy): Redesigning is easier than creating something from scratch and avatar size is 128x128 which is less demanding in detail in comparison with other Artist tasks (but nothing prevents you from drawing in big canvas and shrinking it later).
Deck Builder TFO (below average): Almost every fully upgraded deck can beat HBs and the Trainer can be used to test stuff very fast; so, only the thematic part was the challenging one.
Forum Expert TFO (average): Editing formatting code is easier than creating it from scratch.
Card Designer TFO (average): CI&A image creator tool makes it easy to create 2 card ideas in an hour, but basing them upon OldTrees and Drake adds an extra edge of difficulty.
Off-Topicker TFO (above average): Though it looks like an easy task, it requires a very good sense of humour, good taste and rich imagination.
Writer TFO (hard): Having to write lyrics for your Boss within 100 words (while Writers normally have no word limit) and trying to figure out the possible lyrics of the other Bosses in order to outwit them looks like a hard task to me.