The technology stack you are using is not going to affect the chance of getting into the network. Once you have gone through the screening process, you're will be matched by a team of engineers to prospective clients based on your tech stack, availability, time zone and other factors as well.

When it comes to technologies being in demand, it really ranges all across the board: from the standard PHP, Ruby, Javascript etc, over more bleeding edge like React, React Native to niche things like OpenCV are all there.

Full disclosure: I am currently employed as Head of Community for EMEA @ Toptal