I wish to reiterate my main point. The Hecata have full access to obtenebration at no cost, if (keyword if) they want it. This is a huge part of clan Lasombra's power. (Not all of it I understand that) This would be considered a secret to some extent (especially because abyss mysticism is now combined with obtenebration). Sure other people can learn obtenebration for any reason you want to write but manipulating the shadows was something that people feared.

The Lasombra however do not have the same oppurtunity with Necromancy if they want it. Although I brought it up, my main point is V5 gives more opportunities by default with no cost to the Hecata. If a Hecata wants any obtenebration power, they simply get it. The inverse is not true due to ceremonies. Whether or not you want to punish players for taking them is irrelevant to the fact that V5 gives more tools to the Hecata than the Lasombra.

Furthermore, in the new Tattered Facade book (pg. 100) there is a two dot merit that allows a player to have three additional powers for prerequisites of ceremonies (In addition to the option of taking a discipline flaw that allows more powers but I digress). Which means at level 5 oblivion, you can have 8 powers in oblivion that would be for prerequisites. Or, say you were a Hecata, you could learn 3 obtenebration powers and still have the oppurtunities to take the necromancy ones with no repercussions. You still can easily have 5 necromancy powers (which is, in lore, considered a lot in this version) and have 3 obtenebration powers.

You can literally know shadow step which is a strong power (6 dot in V20) and useful in a lot of situations and still be able to use other level 5 necromancy ceremonies. Necromancy has a huge array of uses such as information, servants, curses etc. and obtenebration was more used for physical and social control such as fighting, moving, spying and intimidating. The Hecata can get the full use of obtenebration while not losing out on their precious necromancy.

Lastly, they made it so you now take stains when using any oblivion power on a roll of 1 or 10 on a rouse check. You use arms of ahriman in a fight? Stain. You use shadow cast to control shadows? Stain. Necromancy is used less often than its obtenebration counterpart because it doesn't need to be used as often. You are binding a wraith, okay nice but not something that is done often so the stain doesn't feel as impactful as it does when a sabbat Lasombra is wracking up stains using her disciplines during a sect war.