Biggest challenges when scaling Dynamics 365 implementations?

I’ve been seeing a pattern across multiple Dynamics 365 projects where things work fine at the start, but start breaking down as the system grows.

Some common issues I’ve noticed:

Data model not scaling well with new requirements

Too many customizations causing maintenance overhead

Integration complexity with external systems (ERP, APIs, etc.)

Performance issues as data volume increases

Curious to hear from others:

What challenges have you faced when scaling D365 beyond initial implementation?

What decisions early on made things easier or harder later?

Any best practices you follow to avoid rework after go-live?

Would be great to hear real-world experiences rather than theory.