I don't know about all that, but I don't think anyone doubts Shiryu is lethal or will disappoint offensively.
The issue is when Garp was fighting Kuzan and the rest, Oda could not even manage to draw Shiryu just straight up attacking Garp, Shiryu felt the need to attack Coby, that's the only way he could have a say in the fight.
Initially I expected Shiryu to cheapshot Garp while he was focused on Kuzan, the actual outcome is a lot worse for his combat abilities, not only was Garp able to detect him, but Shiryu had to go through Coby when he already had an island and number advantage.