Here's a calc that will destroy Mega Sceptile's usability in OU in every way: (Assuming a 20 boost to defense and none to HP, as they never give HP to megas)
252+ Atk Mamoswine Ice Shard vs. 4 HP / 0 Def Sceptile: 408-484 (144.6 - 171.6%) -- guaranteed OHKO
252 Atk Life Orb Weavile Ice Shard vs. 4 HP / 0 Def Sceptile: 374-442 (132.6 - 156.7%) -- guaranteed OHKO
And for a Modest Nature Lapras:
0- Atk Lapras Ice Shard vs. 4 HP / 0 Def Sceptile: 160-192 (56.7 - 68%) -- guaranteed 2HKO
Hell, even taking into account say a 30 boost to Sp.Att for Sceptile:
252 SpA Life Orb Sceptile Energy Ball vs. 252 HP / 4 SpD Lapras: 408-484 (87.9 - 104.3%) -- 25% chance to OHKO Anything less won't even have a chance to OHKO the typical Lapras.
And if it's an Adamant Lapras?
252+ Atk Lapras Ice Shard vs. 4 HP / 0 Def Sceptile: 252-300 (89.3 - 106.3%) -- 31.3% chance to OHKO
Mega Sceptile gained a very bad thing with it's dragon subtyping. Unlike Meganium, it lacks the bulk to pull off a Grass/Dragon typing. Mega Meganium will probably get Thick Fat whenever it mega evolves as a Grass/Dragon too.
A 4x weakness to steel, fighting, ice, dark or ghost makes a pokemon lose a TON of viability due to Bullet/Mach/Sucker Punch, Ice Shard, and Shadow Sneak.
Thankfully the only thing 4x weak to Dark/Ghost is hoopa, I believe.
None of these will be able to switch into Sceptile so who cares.