These are fair and reasonable arguments. However, there is one small fact you all forgot. In a team of 6, how can you carry ALL of the pokemon you need for threats? Not to mention that all these so called "counters" have massive flaws that can easily be dealt with before sending Sylveon out to ruin your day. Not to mention that in a 3v3 battle, predicting whether your opponent will send their Sylveon out or not is a frustrating feat. Lets break down the counters, shall we?
Mega Scizor:
I have one of these, but everyone seems to have HP ice or Fire, so Scizor seems to go down to just about any prepared special attacker, or any physical attacker with Fire Fang or Fire Punch. If I had one of these in my Party, My opponent will most likely have a Pokemon with a fire move to rid of this pest before sending Sylveon out.
Metagross/Mega Metagross:
Knock Off, enough said. Everyone seems to have a Pokemon with this move.
Mega Venosaur:
Talonflame + Brave Bird + Gale Wings. That's it, I'm done.
Jirachi:
1. I don't have one.
2. This Pokemon is banned in Rated Battles.
Heatran:
I have one, and possibly a good counter. But that 4x weakness to ground. . . . Another common coverage type.
Ferrothorn:
1. Same problem as Scizor.
2. Don't have one.
Amoongus:
This is a really sorry counter. It just plain sucks, and so many other Pokemon can easily kill it off. I like the Mega Venosaur idea 100x better.
Charizard X or Y
You are taking up a mega slot for Sylveon? Really?
Talonflame:
Rock moves or Stealth Rock .
I have a Talonflame, but people know better by now.
Excadrill:
This Pokemon has always been a joke to me.
Victini:
Same Problem as Jirachi.
Chansey:
Toxic.
Gengar:
Read my earlier post, this thing just wrecked a Gengar,
Lopunny:
Srsly?
Bisharp:
Who trains a Bisharp?
Hoopa:
Another banned Legendary.
Volcarona:
Left in the landfill with Charizard and Talonflame.
Magnezone:
Shares it's grave with Heatran.
Mega slots taken up for a sake to support/counter a single Pokemon isn't a good argument, IMO. A good portion of these Megas are even good in their own right, as seen with the Charizard forms.
Bisharp is only a
check to Sylveon; it's not a counter if it can't switch to any one of its moves, particularly Hyper Voice:
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 4 HP / 0 SpD Bisharp: 375-442 (137.8 - 162.5%) -- guaranteed OHKO
However, Assault Vest variants can:
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 192 HP / 0 SpD Assault Vest Bisharp: 250-295 (78.3 - 92.4%) -- guaranteed 2HKO
Learn more about the difference between checks and counters here:
http://www.smogon.com/smog/issue32/checks-and-counters