Neb Seni also gives access to the Dragon Splinter cards which these don't, and also these cards are Legendary vs Neb Seni who is rare (I know that doesn't really apply to Neb Seni since he was a KS exclusive card, but nevertheless higher rarity cards are supposed to be more powerful).

Also it doesn't make sense to look at things from the perspective of just one ruleset. The point is to balance the game overall meaning that some cards are over-powered in some rulesets and under-powered in others. For example, while these cards will be amazing in "Back to Basics", they will be useless in "Silenced Summoners".