On the other hand I've seen JavaScript everywhere slow down back end development and increase costs with not much increase in contributions across teams.
I've seen the same. It starts off faster and then as the business grows and features are added the problems scale up. Some symptoms I've seen on projects I've been asked to look over include but aren't limited to throwing Redis around when an in-memory structure would normally suffice because Node is too slow, scaling to 200 CPU's and using up at least 200GB gigs of RAM for a single servioce, when a 25 instances with 8 CPU's, 25GB of RAM could do the same job and cost a lot less, scared to write their own data structures/algo's that may be required as their business scales, etc.