Hi Kevin,
We have checked the shared workbooks and here are the feedback for you.
1.It will pop out a warning window and say that there have too many objects in this workbook which needs to be opened in desktop app to see all content.
2.We find a related official article referring to the similar error message. It says users may not be able to edit the workbook in online as some features are not supported by Excel Online. You can have a check: Edit a workbook that contains features unsupported by Excel for the web - Excel (microsoft.com)
3.Besides one of the workbook you shared is macro-enabled which is also mentioned in above reference, we also find that there have too many pictures like 200 or so in the workbook. Another one have many formulas but it should not be a big deal which cause the issue.
In this situation, please try to edit the original file to reduce the objects to check the result. Also, please confirm with the available users if they could always work with the workbook in Excel online. If so, let them upload the files to the same SharePoint library and see if other users will also be able to work with the newly-uploaded workbooks.
If the issue persists, you may need to submit a support ticket in Admin Center to let engineers here to do a remote check on your local environment and further investigate the issue as engineers here are focused more on app side or widespread issue and may have limited resource to share more suggestion to specific files.
Appreciate your understanding and take care.
Best Regards,
Mia