dex 18 is not zero combat ability. nobody is currently using the katana, and while I am not entirely sure how this gauntled-stuff works, it was implied with huge a mind stat, it would be absolutely devastating against the angels
The katana method suggested was a one-shot deal (and normal attacking would fall prey to Will's minimal Strength and nonexistent relevant skills), the gauntlet is effectively such, and Will doesn't even have a little pistol.
Face it, Will was not built for combat. Maybe sniping, which he did to great effect back in the crossover, but again no gun.
The Katana's power runs on Mag stat, which Will has 0 as.
Actually, it doesn't. Kyle was enhancing his strikes using MAG, but it's actual ability is entirely MIND-based since unlike the gauntlet, it doesn't rip any more holes in reality, it just changes its form for a split second. If he gets a high enough roll, he could probably make it weave and slash in and out of the painting.
Also, I should note a mechanical difference seeing as equipment being made by Cyrielle is used. Since she uses theurgy (calling upon an external power) rather than true magic, it's near impossible to mass produce her equipment easily as Fate has no obligation to have those pieces of equipment remain consolidated. Though in return, Fate magic is near useless against her not that it matters very much (well, aside from friendly fire) seeing as there is usually only about one contractor active at a time.
This is more or less why Creation and Void entities can't use the gauntlet properly, because the power has no obligation to actually form a beam, instead, it follows its natural reaction to rip into Creation/Void. Backfire is when the power doesn't go through the gauntlet and tear into the universe in front of it but instead tears into its user.