Mega Sceptile has it's merits. And interesting STAB. It outspeeds and has a 25% chance to OHKO mega Ray with Dragon Pulse.
And has a 50% chance to do the same to Primal K with Leaf Storm. But it's not a certainty with either of those.
Speed control is everything. Now more then ever.
Tailwind and trick room are literally everywhere. Talonflame is still seen and while quick guard from Sceptile stops brave bird. That leaves it open to Talonflames partner.
+ while you go for mat block with Greninja ~ An interesting concept btw ~ your opponent will most likely be using dark void/tailwind/trick room/rock polish/geomancy.
Primal Groudon cannot be OHKO'd by any of Sceptile's moves. Even HP Ground is only a 3HKO and OHKO's you with Fire Punch / Eruption.
Xerneas isn't OHKO'd by any of Sceptile's moves either. Can go for a Geomancy and the next turn outspeed and OHKO you. :/
I'm not saying don't run sceptile. It'd be awesome to make a team around it. Just be prepared for the fact that it's difficult to use and it lacks the power to reliably OHKO most things.