Thanks to everyone who pointed out what Aran probably really meant
My revised reply then:
4.5 (it seemed easy at first, but got surprisingly hard later!).
I've done Course 1, Course 2, Course 3 superficially, came back to it and started off doing it more seriously but never finished.
Did Level 1, and about 10 challenges of level 2 so far. Had also done a bootcamp before Level 1 appeared I think.
Initially found Course 1 difficult (4/10), but after the lessons 6 hurdle, it got easier and easier (8-9/10).
Found Course 2 slightly difficult at first (6/10) - seemed a bit of a gear change, but then like C1, it got easier and easier. (8-9/10)
found Course 3 difficult (5/10), partly because I was doing it on the move, and not hearing the endings of the short forms properly. I suspect if I did it again now, having had more experience of short forms generally, it would probably be no more difficult than Course 2.
Found Level 1 very easy to be honest, but I don't know if I would have done if it was my first SSiW course. 9/10
Level 2 is a slight gear change upwards, but not really difficult so far. 8/10
I didn't think my working memory was brilliant but about average, but it must be worse than average to only get 4.5.
Having said that, I think it must be easily enough to cope with the SSiW approach, because after the initial steep learning curve, I usually didn't have problems remembering what Aran and Catrin were saying. (Not so easy to remember outside the lessons, but I think that's a different kind of memory, not working memory. Long term memory I suppose or at least long-ish term.)
So perhaps the SSiW method doesn't actually need a brilliant working memory. Just a functional one.