<pre-xmas-rant>
If one use plymouth with siduction (for what reason ever, crypto etc) and the machine behave strange, please don't ask here for help, the only answer will be: PURGE THIS FUGLY SHIT!
Really, flood the debian and ubuntu bug tracker with any meaningful and -less bug reports, but don't ask here, we are not interested in and not willing to do anything else as strongly suggest purging. The DD in charge is vorlon aka Steve Langasek, long term debian developer, ubuntu release manager and the only person on earth who believe that plymouth does everything right.
</rant>
Disclaimer: It may be that i'm a little bit biased, but plymouth was the main reason (among the state of mdadm and other packages) to leave ubuntu. Please help hinder the ubunteros to put their crap into debian. Thanks.