The releases being out of sync is not great, but if you're on Windows you download tortoisehg with a compatible hg, and if you're not your distro's repositories should ensure they don't push a new hg to you before tortoisehg is updated. I went and looked at the history, and the average delay from a mercurial release to the corresponding tortoisehg is only about 3 weeks. Not a big deal. On Arch Linux since tortoisehg isn't in the repos it's a slight pain to hold back mercurial. Hopefully after the Python 3 transition tortoisehg makes it into the repos.
...though if there is not mercurial hosting available by the time bitbucket sunsets support, then maybe I won't care. I really want to keep using mercurial for my public open-source projects.