Because a system is cloud managed doesn’t mean you’ve given a third party company full control of your network any more than trusting that the firmware installed on a Cisco/Aruba/Huawei device doesn’t provide a back door to your system for that organisation.
No user data passes through the Meraki cloud it only used for control, and Meraki have very strict controls and procedures in place about who has access to what. You can find a great deal of detail on this at https://meraki.cisco.com/trust. It is also possible to completely block access to your organisation if you believe it is sensitive:
I understand that this is a big step for some people, but I’d say it’s less of a risk than storing data in Microsoft OneDrive/M365, AWS, Azure or GCP - where the hosting company potentially has access to real information about your company and what it does. To that end I know that Government organisations in a number of jurisdictions trust the Meraki network platform, whereas they have specific M365 environments (deployed by Microsoft) specifically for Government and their agencies to meet their data storage requirements.
All up every customer should understand how their network is controlled and where their data is or isn’t stored, but please make sure that what you are communicating to clients is accurate and valid.