The devilsaur pet can't keep up his own mortal strike debuff even with 100% uptime whilst the Scorpid / Carrion Bird / Riverbeast have no problem with this. I believe this is a bug/oversight and i'm hoping we could maybe get blizz to fix it. I mean all they have to do is change the CD from 8 to 6 or make it apply the same debuff as the others (see thread for details on this)
So yeah, thanks for reading and i hope we can give this issue some attention.
P.S/TLDR;: Don't use the devilsaur for now, it's weaker than the other MS pets.