Most personal CRMs are built Google-first, with Apple support added on afterwards - a one-way contacts export, or no real calendar sync at all. Here's what genuine Apple support actually looks like.
A lot of tools in this category can technically import a vCard file once. Far fewer keep your Apple Calendar in sync both ways, so an event you add on your iPhone shows up automatically, and one you add in the app shows up on your iPhone too.
There's no consumer OAuth for iCloud Calendar the way there is for Google - genuine two-way sync has to be built properly, using an app-specific password over CalDAV, rather than faked with a one-time import.
Built on CalDAV with an app-specific password, since Apple has no consumer OAuth for this - live, two-way, both directions.
Import your iPhone address book in one go, so you never have to transfer it across by hand.
The Integrations page explains exactly what's connected and how, instead of a vague "syncs with Apple" claim.
The Vault is free during early access. Request an invite and connect Apple Calendar and Contacts properly.