While I haven't really tested out Mega Sceptile yet, I think Mega Sceptile should probably get unbanned for the following reasons (yay Mega Absol is getting a retest!):
1. Typing:
While it's one of the many grass-types able to utterly run over Mega Blastoise, it's typing is... not very good. While Grass/Dragon is a pretty good offensive typing, the problem with it lies defensively. It's just so easy to kill. If Golisopod presses First Impression, you're done for. If Metagross clicks Ice Punch, expect to go down - in an extremely painful way. Not to mention that a lot of pokemon also carry Ice-Type coverage to smash grass-types to pieces, all of which have became more popular and are becoming a lot more popular by the minute, and are becoming more and more popular as we speak. And Mega Sceptile is essentially the victim of all this.
2. Bulk:
This is going to be a MASSIVE issue for Mega Sceptile if it ever gets reintroduced into RU. It gets absolutely screwed over by priority such as Ice Shard (oh no...), First Impression, and Extreme Speed. Literally anything bar resisted hits are going to 2HKO or OHKO the Christmas tree. In fact, it's so easily revenge killed, you're in fact going to need to run a scarfer on the set because of how easily the Christmas tree finds itself revenge killed. I'm using a quote from its UU analysis to show what I mean.
"Mega Sceptile is extremely fast and therefore an effective revenge killer, but it should rarely, if ever, be the fastest Pokemon on your team due to how easily it is revenge killed itself."
Yep. You NEED to run a Scarfer when you make a team with Mega Sceptile, because it's burdened with poor bulk and extremely easy to overwhelm thanks to the popularity of scarfers such as Salazzle, Noivern, and Houndoom, so you need something to outspeed all of them, since Mega Sceptile isn't able to do that at all.
3. Hardwalled:
The title says it all: the increased viability of mons which hardwall it are SUPER detrimental for Mega Sceptile, such as Florges, Cresselia, Dragalge, Bronzong, and Goodra makes life much harder for Mega Sceptile. Even taking into account SD variants of Mega Sceptile, it's still possible to hardwall it, examples of mons which hardwall these variants are surprisingly common right now, examples include Forretress, Mandibuzz, and Bronzong.
4. Competition:
If Mega Sceptile was retested earlier, before the grass-types became popular, then it'd be the best out of them. But oh boy, were we horribly wrong. Roserade, Virizion, and Shaymin give it a good chunk of competition, namely, not taking up the mega slot, meaning that you can run a variety of items, having notably better bulk, offer more utility to a team, don't rely on as much support as the others, and are just more versatile in the fact that they don't suffer from a forced item slot. Of course, it does have its advantages, such as a solid amount of immediate power and a notably better speed tier, but what I'm saying is that there are mons that offer more utility outside being a revenge killer. And those three I drove home were examples. I can list more.
Again, I haven't tested it out yet, so this may be subject to change, but I think that the stage that this meta is in gives Mega Sceptile its advantages and disadvantages, so it could be a nifty pick provided it's not broken.