>>280468317
His armor isn't that bad. It isn't the best (the purpose is that it has Anti-Petrification and other spell protections) but it can protect at the expense of breaking from powerful blows, which he uses to gauge opponents.
The main issue really is the arrogance. It wasn't as big of an issue back in FSN when he was meant to be a final boss strongest of all servants (to the point he actually used shit like invisible swords and other tricky weapons rather than just spamming GoB as bullets) but over time it became a more balancing factor, to the point Nasu went on interview to state it could cause him to lose to Saber Alter depending on his master.
Not even FGO, it's Extella that proves that best. While Gil did play along with Altera because everything was under control, he got a ton of losses all-around since he isn't putting effort. To the point in Umbral Star that Altera is seen as the biggest threat due to the sum of power and strategical skill, while in Link he got the most embarrassing display with Karl turned him into a battery.