specifically, features that expand the spell lists of an class (usually two per spell level, and no spells above level 5) such as the background features that add spells from guildmasters guide to ravnica, the spells of the mark feature shared by all dragonmarked races in ebberon: rising from the last war and the expanded spell list granted by every warlock patron considered power-neural in as such as they can only ever add versatility or flavour to an class, never make it out-right more potent? 

They are mostly flavour - but there are some that definitely make a subclass more powerful. The Fiend Warlock for example gets Fireball - one of the most potent low level spells - and Stinking Cloud. At the same level the Great Old One gets Clairvoyance and Sending - two very situational spells - especially given that the Warlock gets very few spell slots to begin with.

I think in general the spell tables even out but there are certainly more powerful options and weaker options between different subclasses.

The warlock expanded spell list is less of an issue since they are not given the spells for free, it only adds more options for them to choose.  I do wish some other classes/subclasses had expanded spell lists.

For the most part the expanded spell lists that give characters more spells to pick from are power neutral because not every character will pick those spells, or want to pick those spells. The expanded spell lists that give characters additional known spells that they always have where other sub classes don’t have additional known spells are not power neutral.

Expanded spell lists are not power neutral, but they are relatively low power. Often nearly 0, but more options can't really be considered 0 power.

Expanded spell lists that are not limited by class are more powerful that the warlock's because it can for example add healing, damage, support, or utility options to a class without it.