Having worked on several projects in the past where the clients had either their own frameworks, and or proprietary languages I think familiarity is highly over

Having worked on several projects in the past where the clients had either their own frameworks, and or proprietary languages I think familiarity is highly overrated in most (but not all cases). If you're building something trivial or basically throwaway then familiarity does play a role, but if it's non-trivial all the other costs outweigh any initial learning.