@tdtechlinkI think it will work fine with 5.x.y because it’s the same major version (theoretically, it should be fine).
The problems may happen when upgrading to Angular6+.
One example is the case I cited above.
Angular6 uses rxjs6 and ionic3 uses rxjs5, and both of them are incompatible.
You can use rxjs-compat to solve this specific case, but it might have other incompatibilities (aside from rxjs).
I don’t know if they would really be incompatible for other features, but for a production app you should pay attention to possible incompatibilities to avoid unexpected errors due to that.
This is a comment from someone from the ionic team about it:
Other posts:
In most cases you see people saying to use Angular5, but in some cases you see people saying that they where able to use Angular6. Just be careful if you use Angular6 with Ionic3 regarding possible incompatibilities.