the point of being Arts, is that you cant do a Burts Chain NP >NP-B-B
>Chain Combo Buff
>Buster Buff
>Prelati Rage 40% Buster Buff for 5 Turns on a 5 Turns Cooldown
>Mad Enhancement EX 12% Buster Buff
>100% Attack Buff at NP5
about the Def, there are many way to remove Debuff, from Servants skills to CE like Vessel of the Saint (Irony being a Jeanne CE)
His buster card only hits once. Maybe if it hit several times it'd be better. Also having to use a CE to negate the debuff is bad too. It only is good on Jeanne since she's support and not about dealing damage.
He also has low attack to begin with. If you like him more power to you but a buster NP would only elevate him to maybe mid tier.
I have seen level 100 Gilles in JP so he does have a fan base