Accessibility in competitive games

In a competitive, multiplayer game, unlike in a single-player game, it's much more difficult to assess difficulty or accessibility. I think the only real way would be to guage how frequently newer players lose to more experiences players. So for example, on average a player who has played Go for 5 minutes (just enough to learn the rules) will lose a lot to someone who has played for 2 hours (enough to actually understand how the rules play out in a game, but not enough to have any grasp of strategy or tactics).

In Starcraft, the 2 hour player will also demolish the 5 minute player. By how much the new player will be demolished in either game, I have no idea. If I had to guess I would say the starcraft newbie would lose more. This is because in Go you will always make a move on your turn, and you may accidentally make the right move for completely wrong reasons. In Starcraft, the newer player will be making fewer moves overall and will lose as a result.

For a similar reason, I'd imagine the new player would lose more consistently in BW than in SC2.

Now I want to address your description "In SC2 after a weekend or two or ten of playing you grab the build order and are good to go". I think this is probably fallacious, but I'm not entirely sure what point you're trying to make, so I could be mistaken. If you're trying to assert that for new players time is best spent learning a build order, then I will not dispute you and you would be correct. However, I don't see the relevance of that assertion to the discussion at hand. If you're trying to assert that by learning a build order a new player is going to be able to compete at a high level, I think your assertion is entirely false.

As I have already described, in a competitive game it is not about how well you play, it is about how well you play relative to your opponent. This makes any notion of a game being "easy" pointless. The only possible relevance would be how consistently the better player wins. In BW the better player wins more frequently than in SC2 (I think) so you could say BW is harder. However, your point about learning a build doesn't seem related to this at all, so again I'm not sure what you were trying to say.

To your chess example, regarding achieving a mediocre elo, that would be an indication that the median competitive chess player has been playing for a couple years, and says nothing about depth or difficulty.