Ok link bait title out of the way obviously Gun Mages have their uses. Let’s roll through the units from worst to best and talk about why these guys op are on the decline as a feature in the all comers list. Obviously no unit is bad if you have a good plan and matchup so we deal only with all comers lists.
Pistoliers are, in my opinion, the worst gun mages in an all comers list. 7 accurate magical true sight shots with ammo types for removing support (snipe), getting you CPs (thunderbolt) and getting decent attrition in (critical brutal), plus the little used CRA for hard targets, sounds great. Trouble is they are not resilient, the need a focus investment to really sing (deadeye) and pop gun pow increasingly does little more than agitate the enemy. Sure you can stick buffs on things – Kiss the enemy unit, decelerate to resist blast damage or what have you – but you can do that to everything. Not to mention they fail command checks from time to time, making people think spending an objective or two points is a good thing. Used to be that the jack marshalling, plus another 2 point investment in Strangewayes, was a major motivation but with stacking armor increasingly harming the hunters AP gun and the presence of Jakes as a focus battery 3 times better than Strangewayes for only one more point I’m not sure it’s as good as it was even a year ago.
Next is Blazers. Defensively a strict upgrade on Pistoliers. More able to shrug blast, pathfinder and light cav for denying line of sight or grabbing terrain and a longer threat range on the useful attacks. For just a handful more points you get 13 more wounds spread over only 2 less models. You no longer need focus investment to deal damage (both brutal and blessed shots coming in at various points for the job) and you still shred stealth infantry (eleap shots) in most real world situations. They also don’t get jammed up as easily with gun fighter and a mount attack to shrug off light skirmish screens. Most problems dealt with but the utility is low. Blazers are a pow 13 shooting unit with 5 shots most of the time and that is not bad at all but you are still looking to chip in damage rather than take it all home.
Finally the Black 13th. I cannot imagine building a list pairing without these guys. For one less shot than Blazers you get more utility than you could ask for. You keep gunfighter. Your defensive tech is stealth, providing you have clouds or some terrain to shove them in. Oh and they are less than half the price of Blazers. Notable interactions are Ryan Mage storming in melee to stop retaliation, or Mage storming a pod to extend stormwall covering fire, Lynch getting temporal accelerated and throwing down two bubbles of death to stealth (which also incidentally nails cloud walls).
Brutal pow 10 shooting isn’t great. If you are investing in gun mages as an all comers list be prepared to be disappointed with the raw damage output. You are bringing these guys for utility. Reevaluate them in that light and it’s clear that while none are bad per se only the black 13th are good enough to be considered a staple.