It can be much harder. Let's say that you have a zipcode<->address conversion library and you're deciding whether to use it within the web stack (option A) or to create an additional service with a REST API on a separate machine (option B). Microservices!

Option A means that it scales with your web stack. If you have 5 application servers today behind a load balancer and your load doubles then tomorrow you will need 10.

Option B means an entirely new set of servers. Not only do you still need those 5 application servers, you need an entirely new server for handling zipcode<->address translation. Let's say it's maxed out.

What happens when your load doubles then?

Well, you'll still need to scale up those 5 application servers running behind a load balancer, but you ALSO need an entirely new load balancer and two servers behind it for the zipcode<->address translation service.

>Different languages/services have to talk to each other somehow and at some point.

This doesn't mean that you need for them to talk over a network layer.