I would like to use this eclass for testing www-client/chromium without having to fork it into my overlay. One issue I can see is the use of the prefix USE flag without IUSE=prefix. repoman seems to barf on that in ebuilds with EAPI 5. I think prefix will end up being added to IUSE_IMPLICIT. See Zac's thread on gentoo-dev.
I just committed naive EAPI5 ebuild support, ie, all checks relating to EAPI4 now also pass for EAPI5. This does not mean that the eclass shouldn't be otherwise updated for EAPI=5 features such as sub-slots/slot-operators. Also, since IUSE_IMPLICIT's values are afaik not finalized I didn't make the eclass add 'prefix' to IUSE. This should be done if it's decided that prefix will not be part of IUSE_IMPLICIT.