The following limitations are found in modern forms due to performance reason and/or limitations of SharePoint API.

• Maximum number of items in lookup columns - due to performance reason, by default the maximum number of items to be displayed in a lookup column is 2,000. If you need to be able to use more values, use auto-complete mode of Connected lookup column and set the number of type-in character to more than 0. Users will need to type in characters and only values containing those characters will be returned.
• Events list are only partially supported. Specifically, there is no support for recurrence.
• Managed metadata columns should never be renamed after creation.
• Forms work in browser only (desktop or mobile), you will not be able to use them in the SharePoint app for mobile devices.
• Users must have the necessary access to the list as well as additional lists that are used by lookups, associated items, document link columns. Read access to Site Assets and Site Pages (previous versions in SharePoint Online only) is required.