Let's look at the products helping managing oncalls on slack. For the begining, we can compare custom zapier with pagerly and integration with Pagerduty and Opsgenie
So what's the Conclusion?
It is cost efficient.
Pagerly can create oncall bots, whereas Zapier cannot
Through Pagerly, you can override and see all the oncall's schedule within slack itself. With Zapier , it requires setting up complex zap between Google Sheets, Pagerduty/Opsgenie , Slack
Pagerly's deep 2 way Integration between Pagerduty/Opsgenie gives out slack ops functionality over Zapier.
Both Zapier and Pagerly doesn't store any user data.