The Missile Boat was specifically designed to counter the menacing TIE Defender. which when given everything could more than tripled its speed for escaping &/or blitzkrieg-ing its targets), otherwise it was almost perfect for the role(s) it was given: Pretty much the only drawback it had was a lack of ion cannons ( massive warhead payload, good manuverability, stronger shields & hull than the TIE defender & B-Wing, and a top speed equal to the A-Wing before activating the SLAM Drive. Cue the Battle of Endor not long after & it disappeared into obscurity before it could get fully out of the experimental stage.
The only real reason(s) it wasn't seen after Endor was due to the Rebellion almost getting away with reverse-engineering examples of the fighters ( they didn't), but Palpatine got pissed off at the possibility they had them recalled into protective storage 'until the Rebel problem was solved' as the war against Zaarin had resulted in many of its manufacturing centers being destroyed. Click to shrink.He also designed the Missile Boat, which was not only a good craft in general ( as it was made to fight against TIE Defenders one-on-one) its also one of the best heavy bombers in Legends.