Each magic scroll can give up to 3 stats for each head accessory.
The ranges those stats can take are (min-max):
HP 383-803
HP recovery 28-56
SP 76-150
SP recovery 21-42
Physical Damage 21-45
Physical Defense 7-11
Magic Attack 21-42
Magical Defense 6-11
Magic Amplification 21-42
Critical Damage 32-66
Critical occurs 7-14
Critical Resistance 7-14
Hit 7-14
evasion 7-14
Property Damage 15-33
Resistant properties 15-28
Greater Attack Rate 1-1
Greater percentage defense 1-1
speed 1-1
Source
Sure it’s RNG, but if someone buys 100’s of scrolls, then they are practically guaranteed to have some really good rolls (not perfect! But very good!).
Also let’s just say that a possible extra 2400 hp seems like a pretty big advantage in my book. We’ll see what hp looks like at endgame, but for players around lvl ~80 that’s like a +50% hp boost for glass cannon players.
EDIT:
The biggest issue is that it’s cash for stats. Sure, you have a random element to it, but it doesn’t change that basic fact. It also keeps open future insidious possibilities. If it doesn’t get removed, I’m predicting it here, there will be more cash for stats in the future (with some RNG flavoring to make normally rationale people derpy)
Some people argue that you can get magic scrolls from dungeon runs. Based on anecdotes from kToS users, the drop rate is like 1 scroll per every 200-300 boxes that drop from bosses. The rate and amount which free players can get is MUCH lower than what paying players can get.
Paying players will be able to get ~12 scrolls for $10 bucks (if current rates stay the same when TP buying finally opens up). I calculate that if it takes a normal user 15 minutes of dungeon running to acquire a box, then you need something like 50 hours of JUST dungeon running or world boss grinding to get a reasonable chance of 1 magic scroll.