VSTO/COM Add-ins for Office have been used for many years, but since 2014, they have gradually been replaced by the more advanced Modern WEB Office Add-ins. Initially, app developers were limited in their capabilities due to many missing Office APIs. However, over time, the Office API capabilities offered to the Modern Web Add-ins continuously expanded. As a result, developers like officeatwork have been able to shift their solutions to the modern WEB Office Add-in platform. Below are some visual aids illustrating the differences, advantages, and disadvantages of the various Add-in technologies.

Architecture

Pros and Cons

Additional informal provided by Microsoft on this topic can be found here:  https://learn.microsoft.com/en-us/office/dev/add-ins/overview/office-add-ins#how-are-office-add-ins-different-from-com-and-vsto-add-ins