The current ebuilds for anki, 2.1.0_beta25 and 2.1.0_beta27 have some issues, however, those have already been reported, namely in 641966 and 643004. No worries; there's clearly a good reason they're marked as testing. After manually adding webengine use flags for PyQt5 and manually adding PyQt5-5.9.2 to package.accept_keywords, an emerge -u PyQt5 makes Anki Just Work™. Jolly good.
However, it's a little odd that the only ebuilds available for anki are these beta releases of the 2.1 series. It's worth noting that the 2.0 series is the /only/ supported stable releases at the moment from upstream. As a matter of fact, after tweaking the manual flags and keywords necessary to make 2.1.0_beta27 run correctly, launching the program from a terminal displays the following:
"This is a BETA build - please do not package it up for Linux distributions"
Now that's just a bit on the nose, isn't it?
I don't like the thought of only unstable versions of a piece of software being available in portage when stable versions are available. When I wake up, I'll start work on an ebuild for 2.0.49 based on the pulled last-rited ebuilds ending at 2.0.47, and I'm largely using this bug report to reference on the subsequent pull request on github, since those seem not to go anywhere without a link to bugzilla.
2.0 depended on QtWebkit4, which is full of security holes and not in tree anymore.
So, rather than wasting your time on the old version that is impossible to get back into tree, I would recommend to improve anki-2.1 ebuilds in bug 641966 and bug 643004.