I've done some work with salesforce integrations and from my perspective, the pain comes in when trying to keep a bi-directional link between data in the CRM and data in the custom backend systems. At least in my case, there is a desire for a low latency sync so that the two data sources are as similar as possible.

Salesforce, with its batch loads, open api spec, soap/rest api and 'use mulesoft' approach makes this pretty painful, and quite expensive.

The dream, at least among folks I work with, is for a single pane of glass where salesforce provide contact and case management, and the internal systems provide a source of truth and the apis or screens to fix problems or complete tasks