Frigate as a battleline is no surprise at all. Will it be useful? dunno but you already can have 4 of them at 2K (or ironclads). With the new rule (fire from skyvessel) 4 of them, buffed and with troops inside, is already a big investment.
Not knowing the whole scope of the new rules, I don't know yet if I'd go more than 4 but it is good to have the opportunity.
This is not a KO 2.0 battletome, this is a NEW KO battletome, with new warscrolls, mechanics, possibilities, reborn from 2 years of AoS experience. Well done GW for , at least, having the balls to re think your work (avec eventually having listened to you fellow cuwtomers/players)