When comparing Bugsnag vs Fossil, the Slant community recommends Fossil for most people. In the question“What are the best self-hosted bug trackers?” Fossil is ranked 3rd while Bugsnag is ranked 8th. The most important reason people chose Fossil is:
Fossil includes source code management, bug tracking, a wiki, and technotes. It even includes its own web server, though it can fairly easily be incorporated into other webservers.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Great documentation
Clear, concise documentation. Plus docs on cross-domain script errors and enabling CORS.
Pro Email, chat, sms notifications
Bugsnag can be set up to send email notifications as soon as exceptions occur. It can also integrate with a number of third-party services that can be used for notifications including Campfire, HipChat, Twilio SMS and custom solutions via Webhook.
Pro Automatic issues/ticket creation
Bugsnag integrates with a wide variety of 3rd party tools to automatically create issues or tickets. Integrations include GitHub, BitBucket, UserVoice, Trello and many, many others.

Pro Has a free plan
After a two-week full-featured free trial period the user is limited to 2,000 errors/month, 1 user and 1 project.
Pro Exceptions are grouped
Bugsnag groups similar exceptions together displaying the number of occurences so that there's no need to deal with each instance of the error separately.
Pro Exceptional documentation organization
Their documentation was well organized, easy to search through for the platform of choice and quick work of full product integration.
Pro Supports Unity 3D
Drop in place integration with Unity 3D, only a simple API Token configuration on game object and everything was up & running with almost zero code written.
Pro Supports phabricator integrations
Not many products have decided to finally start doing this, phabricator now has a champion.

Pro Source map support

Pro Stack traces
Pro Client libraries are open source

Pro Report uncaught errors
Pro Good web UI
Pro Very complete
Fossil includes source code management, bug tracking, a wiki, and technotes.
It even includes its own web server, though it can fairly easily be incorporated into other webservers.
Pro Self-contained
A Fossil repository is contained in a single file.
Pro Cross-platform
Fossil can run on Linux, Mac, BSD derivatives and on Windows.
Pro Very easy to configure as self-hosted
Single, stand-alone executable, including web server.
Pro Needs very few server resources
Since Fossil is a distributed VCS on top of being a bug tracker, it needs very few server resources.
Cons
Con The tool seems flaky
De-obfuscation doesn't always work on react-native projects.

Con Blocked by ad blockers by default
Bugsnag is blocked by the AdBlock EasyPrivacy filter.
Con Only a web interface or CLI
Fossil's bug tracker only works with the web interface or the command-line interface. There's no native GUI client supporting it.
There are some independent GUI clients out there, but none of them support Fossil's full range of abilities.
