Trademarks: When is a Trademark Considered to be Abandoned

Let's say a company registers word and logo marks for "BoffoProduct" 9 years ago. They then fail to use either with the registration mark, ie BoffoProduct®. However in rare circumstances they do use the TM mark, ie BoffoProduct™. In that period, many companies mention BoffoProduct in a "compare to" context, ie Compare OurProduct to BoffoProduct.

Can the marks be challenged before the PTO as abandoned due to non-use and/or failure to enforce?

Trademarks: When is a Trademark Considered to be Abandoned