It might just be me, but it feels like the Keto is extremely fragile and underpowered compared to other things in it's weight class. The main gun is ok if it hits, but this is countered by the fact that it's slow and cumbersome and it can quickly be stripped of most of it's offensive capability since the various parts of the ship are quite easy to destroy. Also, under player control, depending on the mods you have fitted, the main gun actually has more range than you can actually fit onscreen.
Nemean Lion seems good, though I wish the armored shell was more durable.
The only comparable ship is the Astral (I never balance my mod against other mod!) And frankly, it's power comes mainly from the fact that the missile balance has been (imo) thrown out of the window in vanilla. It used to be a tough nut to crack, but now it vaporize anything that come close too.
I'm not sure I will change the Keto, at least not beyond buffing the gun and flight-deck health a bit. I had a lot of fun playing carrier fleets with that ship as the core, sniping targets at extreme range to create opportunities for my fighters. And in AI control I find it behaving quite well, trying to stay out of range of the bulk of the enemy force. I mean, it's not a battleship, it's not even a battlecruiser: it's a carrier with a nasty bite.
As for the Nemean Lion: free armor + flux-free PD + damage reduction while venting + active PD while venting... You lose some plates easily, but you really have to make a big mistake to loose the whole ship! ^^