Just a brief update, because I needed to get solid code coverage for a decent phoenix app (umbrella app of 5 sub-apps, 17k loc).
On the app side, we’ve been using excoveralls : it is reliable, can be run in umbrella apps, with partitioned parallel testing and can output in different formats. 
Then we needed also a good UI to be able to drill down within our application code to know what to test to improve our code coverage. I tested:
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coveralls.io: the UI is only OK but not really convenient nor pretty. I could not make Github Status checks or comments to work (in order to get instant code coverage feedback when opening a pull request). Integration in the CI was not really convenient as well because we had to disable partitioned testing to run all tests at once and upload a single report.
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codeclimate.com was disapointing
Integration within CI was ok : we had to generate multiple json reports (one per partition) with excoverallsthen format them, merge them and upload the whole withcc_test_reporter(binary provided by code_climate). Github feedback is working well, but you can’t really explore your code source tree within codeclimate UI. No go
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codecov.io is our choice. Integration within the CI is really simple : you can upload multiple report for a single build (one per test partition) and everything will be merged on the serverside. Github feedback is really insightful and the UI gives you the ability to drill down as you want to build your test coverage strategy
