Free private repos used to be the one thing keeping me (and from conversations, several people I know) on Bitbucket and once GitHub offered those as well, there was no incentive to stay. The Mercurial support was probably another unique feature they had from way back but they seem to have decided to remove that too.
Central login where you can never understand what login you are using and what information they are collecting, nor on which website you are not whether you’ve suddenly integrated Confluence into a Bitbucket instance, is a real downside. God I hate SSO’ed companies, they went all the way of Google into Youtube.
which isn't true anymore, as other pointed out.
Also offering Mercurial was a reason for choosing Bitbucket, but they are ending its support.
See "Want a more powerful issue tracker?" at the bottom of https://confluence.atlassian.com/bitbucket/issue-trackers-22... for the documentation of this policy.