My company’s stack is a node.js API with a web app that’s a combination of next.js and a single page React app. Our scale isn’t massive either (~50k active users) but since the entire thing is written in Javascript it means all our engineers can work on all parts of the app. It’s kind of like how some airline companies only buy one type of plane so all their pilots can fly all their planes.
Haven’t experienced any “bottlenecks” but again we aren’t at huge scale or anything and we don’t have any heavy blocking computations where the single Javascript thread would be a problem.