Sion's numbers are wrong, his concept is right.
Obviously the idea is low base health and then he scales up past everyone else. The problem is that in reality he scales up to a mediocre level of health and then the game ends.
Let's look at some typical numbers and then look at Sion's.
High base health gain per level: 90+ (highest is 102) Medium base health gain per level: 80-90 Low health gain per level: 70-80 Mini Gnar: 65 Sion: 70
Sion gains 2 health per minion, 8 per champion or large unit.
IIRC, each lane spawns 2 waves per minute and 1 of them has a large minion, so Sion can get 24 health off the regular minions and another 8 per minute off the large one for 32 total per minute if he has freefarm.
Old Sion had 104 health per level so he lost a total of 34 per level, 544 total. So this means he needs freefarm for 18 minutes to match his old base health or reach similar health to other melee champions.
Getting 100% of the minions for 18 straight minutes is not very realistic especially when you're playing a melee champion with one of the lowest base health totals in the game.
This type of design doesn't fit in LoL because most champions were designed so that their abilities are in addition to normal base stats. Veigar does not give up stats for his AP gain. Nasus does not give up stats for his Q damage gain.
In fact, if you look at champion stats you'll see that they tend to compliment the champion's strengths rather than subvert the strengths. Nasus has one of the highest base HP regen stats in the game on top of his lifesteal passive. Both if these things allow him to sustain in lane so he can farm up his Q and scale into late game. His kit and stats compliment each other.
Meanwhile, Sion's stats make him so weak he has trouble farming up his health due to poor laning. To make matters worse, the scaling he's given to supposedly compensate is so weak that even if he is highly successful in lane (18 minutes of straight freefarm) he will only match the natural base health of his rivals.
What more is there to say? Fix Sion's numbers please.
Health: 420 +70
Health regen: 7,5 +1
AD: 53 +4
AS: 0,679 +1,2%
Armor: 18 +3
MR: 30 +1,25
Health: 445 +80
Health regen: 6 +0,65
AD: 49 +3,4
AS: 0,658 +3,1%
Armor: 21 +3,5
MR: 30 +1,25