The smallest gunboat, the Bount Hunter Gunship is too small to handle cap armor under 8. With 8 she's insane, without 8 she's very very very good. She relies more on size and maneuverability to evade fire.
Above that though gunboats need to be able to suck up some damage. A larger gunboat without cap armor is going to die very quickly at the hands of other caps in addition to bombers.
Cap armor is an investment on a gunboat that lends to her survivability but does not make her invulnerable.
I've squared off against plenty and never felt that a standard gunboat with cap armor is invulnerable.
