When the guns came out there was numerous complaints about how these were the "worst guns ever". So far I have crafted two 5star guns and have played with them a lot, enough to understand their mechanics.
The standard attack is rather lackluster, it does a fair amount of damage but in order to do so you much get close to the enemy. It does however have what could be the best close range knockback of any gun. This knockback works extremely well with the charge attack as it allows you to quickly position an enemy so they take the most damage from the incoming shards.
The charge attack is unique and indeed seems random, however after looking at existing attack patterns I would say that this charge attacks is considered a mix between the Jelly Cube spike attack and the Gremlin Mortafire rocket attack. Much like a T3 Jelly cube the shards land close to/around/on top of an enemy (some randomness here but much like shard bombs you have a general idea of where they will land) and like the Mortafire they aim at the nearest enemy using a form of Auto Target (this is applied whether you have AT on or not). The shards will try to land on/near the nearest enemy, if the enemy is immobile the shards will land beside/in front of the enemy but almost never behind.
This means I can shoot a charge and then use the regular attack to try to push the enemies into the incoming missiles.
Tortogun targets include slow and immobile enemies, fast enemies like Kats, Wolvers, Devilites, Twackers, greavers, etc should be lured into the spikes rather than targeted them.
Interestingly the Omega Tortofist (and any tortogun not resisted by constructs) tears up Mecha knights, When you fire a charge they stop and shield, and right before the shards land they unshield and take massive damage.
Another complaint is the "lack of damage", this gun makes for good area denial, and is capable of doing good damage to groups.
This gun doesn't make you walk slower when charging (like the Blitz needle) and doesn't root you to the ground (blitz again), in fact you can use the movement you get from the charge attack to put some distance between you and the enemy.
While not the most powerful in terms of raw damage this gun is extremely fun to use and has a place in any players arsenal.
These guns are by no means weak.
At max damage, using the Omega TF vs the Nova is only a 2 minute increase in the total time for a vana solo (33 vs 35 mins).
Also, max dmg Gorgofist can one hit gremlin T3 menders in a party of 4 on elite. At D28. (need some luck with the cluster tho).
CTR is definitely more beneficial than ASI, since it is not good in LD. As OP said, Kats are near impossible to hit directly, and its a tossup with wolvers and thwackers.
I find this gun best against mobs, since it can do blotz dmg without the drawbacks.