Thanks! Yes, I used both, and one of the founders of Linear App was a staff mobile engineer at Uber, where we worked together.

Bugsnag is mobile crash reporting and Linear App project management (not specific to mobile, more like an alternative to JIRA). I see zero overlap between them.

AFAIk they are not competitors. Linear is akin to Jira/Trello. Bugsnag is akin to Crashlytics.