Great !

A couple of reasons;
1- You have not included 'jungle' anywhere in the title, and yet your giving a jungle build.
2- Sion is not easily kited- he has the longest ranged (non ult) slow in the game, as well as 2 knock ups.
3- You have only given the starting items and final build- not what to buy in-between, which would be incredibly useful to a beginner (at whom this guide is aimed at).
4- This is probably the biggest one- You have relied on a YouTube video that you didn't make for the biggest part of your guide.
5- I'm not going to go into detail, but I strongly agree with drwh033 on your item choices.

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Its a little discouraging when 90% of the matchup are listed as counters :(

Sion has great ganking potential if your teammates can either follow up with cc, or lock your opponent so they cannot escape. The BC is not always bad, in my guide, which offers different way to build Sion makes the BC very effective as the passive is used well. You can then get off the 25% armor pen, plus the 20% from the Roar of the Slayer, with 45% armor pen, you can melt your enemies.

Nice. Thank you for your opinion.

Umm sorry No.
I don't know in which division you are playing but you won't definitely get these results in ranked games above silver/bronze.

a) sion ganking potential is semi decent but not even close to "insane" as you put it. If Sion has insane ganking potential then what does Kha Zix or Pantheon have? Godly?
b) BC is a bad item for him as it takes ages to put all 5 stacks of the passive on an enemy. He is not an autoattacker and he only has one physical dmg ability Q on a long cs that is easily avoidable and stop your AA while casting. So NO BC is bad. LW might even be better.
c) Ghostblade gives a decent much needed MS speed boost but again is an item mostly oriented for Autoattackers with low AD values.
d) SV passive is totally wasted on him. Go for Banshee's veil instead.
e) Yes all these 3 items are good when Sion is dead. But guess what... In any ranked game above bronze/silver enemies are going to CC the living sh1t of you untill your passive dies out. When you are dead you are irrelevant in 80% of the cases. The fact that you don't build health makes your passive even weaker as a mere 1 second slow is enough to render you useless.
f) Warrior enchant is not bad but still it is inferior to Juggernaught for Sion.
g) Sion can only be played as a disrupting tank with decent CC and ok damage. That is a fact.
h) If you are fed and you have the luxury of building a damage item to snowball I would suggest phage--> trinity force. It has too many useful stats to pass on it and also gives a much needed mobility boost * also needed for passive chasing)