The following are the top three gotchas of SharePoint Online:
1) Modern List Attachments:
List attachments do not have robust management controls and collaborative features such as versioning and co-authoring.
List attachments don’t have a drag and drop aspect.
2) Customization Challenges:
SharePoint has extensive customization options, but what is available via best practices is ever changing and not always documented.
Be aware of customizations that might impact performance, security, or scalability.
3) Anchors in Modern SharePoint Pages:
Anchors allow one to link directly to specific sections within a page. However, when working with anchors in the SharePoint Framework code, this can be difficult.