It can do MASSIVE hate free (or rather hate transffering to ideally tank) damage and if you can stand in sweet spot for guns, then it gets really brutal.
2 things on this idea.
1. Tanks move unless you yell at them not to. Ask any THF. AND mobs have knockback just in case you have a tank that cares about what is going on. So just be a smart tank and sit there sounds great but in practicality they can just complain that you could just use any other strat that's easier for them and just as effective and they wouldn't be wrong. Tanks can keep up with hate generally well enough to make hate transfers unimportant in this game, plus Arise takes the punishment for not holding hate significantly less. There isn't enough incentive in most cases to care about adding an extra layer for a tank unless it directly benefits them.
2. The sweet spot for guns in under 10 yalms and the sweet spot for bows is right outside of 10. RNGs are not renowned for their defense or magic defense. You will die in your midshot set at some point during a fight that you sit that close to any real content. Dead dps = bad dps. No AM3 on an AM3 build = bad build.
My Gandiva glows, but it's future is not bright until at least Empy sets. The sweet spot for ranged should probably be farther back on at least bows to make them more attractive and closer for guns to allow you to spam last stand in melee at max efficiency. Crossbows idc about since I'll be using Trueflight anyways and either using an obi or an orpheus sash to decide where I'm standing for max damage depending on the content.
The math of Gandiva and Armageddon is interesting and points to an obvious winning solution. The reality of enemy moves, poor gearing support, and lack of a problem that needs to be solved by it's unique setup is it's biggest hurdles. If you get past those hurdles, it murders ***dead... Just like a lot of less complicated strats.
If only there was some move called Cover that the worse of the 2 serious tanks possessed that actually helped with this type situation and made that tank more useful. We can always hope that PLD won't get RNGed in the job adjustment department, but I'm not holding my breath because Cover has so many problems that makes it ineffective, just like PLD itself. If nothing else, it would have to cross party since PLD isn't going to use the same buffs as an AM3 built RNG or COR.