Dota 2's arcana aren't the same as League's Ultimate skins, and actually have less features more often than not; the very first arcana was literally setting the character's hair on fire gave her new vfx on ONE ability, a few new icons used in game. Even the first ultimate skin for League Pulsefire Ezreal has entirely new animations for everything Ez did, has multiple models that are changed as he levels, entirely new VFX and a new VO. Even now when there are substantially more features on the arcana they still don't have as much stuff put in to them as League's ultimate skins, which is why we have had three over League's six years of being released and Dota 2 has eight arcana in its three, arguably five years of being released.
Me even comparing them is flawed anyway because it's like comparing apples and oranges. They're different products made for different games. I mean hell the Arcana are made of multiple components you have to equip to your hero, not a singular skin like in League.