We have several projects hosted in Azure Web Apps, both CMS admin and frontend. We don't have any issues with the scheduled tasks running inside Admin app, rather than configuring a dedicated service.
Yes, scheduled tasks documentation says
Use the Scheduler Windows service for supported tasks if possible. The external service always runs and executes tasks reliably.
But it is possible to make the default in-admin scheduler the same reliable - all you need to do is to enable "Always On" for your CMS admin App Service in Azure, and that's it!
I've just checked the documentation and for v13 "Always On" on App Service is enough since CMSUseAutomaticScheduler web.config key is now "true" by default. Historically, for previous versions this web.config key was set to "false" by default so a few additional settings were required to make the default scheduler more reliable.