Ask why a technology programme slipped and the answer is almost never that a platform failed to install. Installation is the well-documented part. What consumes the schedule is the space between systems: certificates that do not chain, identity attributes that do not match, two products that both believe they own the same record.
Integration is treated as a task at the end of a plan when it is really the plan itself.
The boundary is nobody's default responsibility
Each vendor is accountable for their own product working. None is accountable for the interface between two products, which is precisely where the difficulty concentrates. When something breaks across that line, the honest answer from both sides is that their component is behaving as designed.
The fix is contractual as much as technical: one party has to own the boundary, and it has to be written down before anyone starts.
Name every interface before you build
A useful architecture document lists interfaces explicitly — what data moves, in which direction, how often, who owns the record, and what should happen when the other end is unavailable. That last question is the one most often skipped, and the one that determines whether a failure is noticed in minutes or at month end.
Interfaces discovered during implementation are always more expensive than interfaces designed before it.
Test the failure, not just the success
An integration that has only ever been tested on the happy path is untested. Proving that a message retries, that a duplicate is rejected, and that somebody is alerted when a queue stops draining is worth more than another demonstration of a record moving cleanly.
In short
Design the boundaries first, give them an owner, and test how they fail — not only how they work.