Maybe it's just better to not work at these kind of. Dry low technical skill SF "young web" startups. I dunno.
I think it makes sense in a way. If your technology is built on Node then hiring a guy who'd rather work in php or java is not going to be a good fit. He won't go home at night and play with Node to really understand it's strengths and weaknesses and he won't have excitement for the technology -- totally justifiable since at some point all these new frameworks start to feel like re-inventions of the same wheel over and over again.
In my experience there are nice things about working for SF startups --
You're surrounded by people passionate about technology.
You're generally working on problems that are small enough where you can have big impact on them yourself.
If you want to know how to build a company then it's really good experience.
That said, it's sort of a question of what scale you'd like to work at.
In my experience --
Contracting is fun because you build lots of small stuff and experiment with lots of different technologies and ideas
Startups are fun because you get to actually build and run a product but you have to build everything so sometimes you don't get to venture into those really interesting areas like massive scale or search quality
Enterprise (I haven't done a ton of enterprise work) seems fun because if you're part of the right enterprise then you get to work on problems that are much larger than a startup can work on and work with more resources and more exotic larger problems (wouldn't it be fun to work on self-driving cars?)
Well, in a month or two, what do you get? Who knows. Evaluating anyone from a resume is very difficult. They might be terrible, but you had to give them two months to find out.
If they claim to know the stack, a bad hire has nowhere to hide. They'll be found out much quicker, so you've lost much less.
I tried to answer this, but it boils down to this. No one is going to say "I'm looking for node and boot campers because I don't have a lot of money."