I feel Seraphim is kinda weak when it comes to mid game air, and also nothing special in the late game as well. in the t2 stage they have a good t2 transport, but the Notha and Vulthoo are nothing special. I'd like to talk more about them, but to be honest it's hard to remember when I last saw them, so I don't think I can comment further. Edit: Well I saw them yesterday, in a high rank ladder match from a player who had full air control. And that player lost!
In the t3 air stage, they get a Strat bomber, ASF, and Scout that are all pretty much the same as other factions - except that their bomber has high damage low aoe (and no aa but that doesn't really matter). What they don't have is t3 gunships. Broadswords, Wailers, and Restorers are strong and versatile. you can basically use them as many times as you want because they survive a long time and heal for virtually no cost.
Lack of a t3 gunship isn't fatal, but I think we should buff the Vulthoo to make up for it. Once again I'm not sure about what exact values, but I think making them more of a concentrated gunship (higher power, higher price) would make Seraphim Air pose more of a threat.