well- typing does not necessarily make it uber or not, it is the over all playability it has for a team whether it supports or is an attacker does. Your opinion is a good one on how I see it as for the typing of grass + dragon at a first glance and generating my first thoughts to the typing in the first place.
What moves the Pokemon has available to it and combined with a held item and it's ability will make it or break it.
At a first glance- not so great.
with this:
Lightning rod ability will divert electric attacks away from partnered Pokémon plus raises spec att one stage. Sceptile is currently a spec attacker (as per the higher base stat vs phys att base stat)- so a bonus there. (this goes to say for current and if the megavolution gets a spec att base boost up grade and remains a spec attacker)
Dragon is weak to ice and fairy, grass is ice and fire. Fire will prolly be bumped to normal damage, ice still being the biggest threat of course. And poison will do as much a threat as ice, so that is the main 2 threats right off.
Sceptile currently out speeds Charizard, which Charizard is a big slot filler on current teams. So, Sceptile could still out speed Charizard in the new games.
Sceptile currently has a few dragon attacks and some rock attacks as well, plus- even though Grass Pledge is over looked: it could work well for double in a big way after mega evolving with a partnered water type pokemon.
there is a lot to take in to consider how to use a Pokémon regardless of what's it's weaknesses are, there is strength in there somewhere if you really take the time to look pass a serious fragileness (ice and poison for type in megavolution) to what is currently a huge threat in the meta vgc right now.
it is early to get to far ahead in thinking on how to use Sceptile until it is closer to the release time of the new games.
All I know is that for the time being- I am already working on some Pokémon and combos and the what not and building them now in possible preparation for the next format.
I have always liked and have had great success with Sceptile back in the day, and even with adding dragon type to it and the lightning rod ability opens an old "battle worthy" Pokemon for the starter returns and a 100% 2014-15 format user is going to mean new changes.
so you all know, no actual real facts in some parts of this post are to be taken like it is going to be that- speculation, what is current, and some opinion is most of this post's design.