But do they attract readers? When I research topics for WP articles, I often come across existing WP links and can't remember seeing any links to even major WP forks; this is the main problem I think Epistemia faces- penetration and hence credibility.
Precisely, that's the problem. They need to attract readers and aren't doing so. The way I see it, there are two ways they can try and attract readers, they can be better or they can be bigger. Being bigger isn't practical when enwiki is approaching 3 million articles - it would take years for them to catch up even if they could attract 1000s of dedicated writers. That only leaves being better - they need their articles to be significantly better than Wikipedia articles so that people see the point in looking to see if they have an article on the topic rather than just going to Wikipedia knowing we're sure to have one even if it isn't particularly good. Writing stubs isn't going to move them towards that goal.
Whereas I realise that what might be loosely described as "market penetration" is important, I think any endeavour trying to compete has great practical problems to overcome. That's not to talk down the idea itself, just to point out that when the hill is already so high, attempting to climb it is that much more difficult. It's unfortunate that the hill is somewhat littered with rocks in places, but conversely, nobody seriously believes the earth is flat these days.
Indeed, but competition is good so I wish them all the best, even if the challenge before them may turn out to be impossible.