The Tankgewehr certainly isn't for everyone. As others have noted, on its own it's a slightly underwhelming 5XP weapon. However, when you look at who can take it and what else they can take, it starts to get very appealing as a synergy weapon.
This is Dexter Drake's favorite gun on the Citadel. Molly can pull it out of his hat with flawless accuracy. He can do Great Work In the Thick of It with Charon's Obol to Delve Too Deep and have his guns on the line by the second scenario. The tank gun is competing for the affection of Dexter's hands with things like a cane, some candles, or maybe an atlas or flute; even the good books, scrolls, and gang signs don't tie into his love 'em and leave 'em relationship with spell assets. Against those, it looks respectable on its own. Then his mafioso man Friday shows some Quick Thinking to help him defray that extra action cost. Because "Activate" is a type of action, he can even shoot off one of his spell assets, then shoot off his gun, and reload with a quickness—his face is even on the card. And when worse comes to worst, he can burn one of the baddies under his Binder's Jar to cancel the hit from a badly timed reload.
Speaking of Leos, Leo Anderson is right up there with Dexter on the list of guys who love this gun. He has all the same rogue tech and economy as Dexter to support it. He may not have Molly, but he has the classic line of Guardian gun tech: Prepared for the Worst, Marksmanship/Telescopic Scope, Custom Ammunition/Custom Modifications. And he can go On the Hunt or take First Watch to make sure there are plenty of things for him to shoot (in addition to the rogue moves for that). He can let his dogs tank the hits from inopportune reloads or call on cats to get him out of the situation. The apex of this gun is when Leo quicksilver snipes a big wiggly squid face or a sticky bubble bath in the next county, then reloads right in front of a doom-spewing science project (behind his Bulwark), letting him follow up with a hasty no-scope.