For Multiexcerpt - Cloud: You cannot add a Multiexcerpt macro or Multiexcerpt Fast Excerpt within a Table cell. The macro gets created outside the table when you try to insert it into a cell.
This happens only in the new fabric(V2) page editor. It works for the old editor. This is only an issue in Confluence Cloud.
Multiexcerpt Includes CAN be nested in tables, it is only Multexcerpts that cannot.
Atlassian has announced that they will NOT remove the legacy page editor from Confluence so you can use that editor. : Attlassian says, "The legacy editor isn't going anywhere"
Rich-text body macros can be nested in Tables in the legacy page editor.