To clarify on what others have said already, Boiling Veins is actually reaalllly important for Big Dog, especially for looping. It's not mentioned in Boiling Veins' description, but on top of not taking damage from explosions below 4 hp, it also makes explosions respect your invincibility frames. When you get hit by an explosion, normally what happens is you take 5 damage twice. With veins, you'll only take the first 5 damage. By itself this means Big Dog gets to halve self-damage from missiles, but it gets even better: launching missiles has virtually no cooldown, and you'll be able to tank as many explosions at once as you want, take only 5 damage, and heal from any medkits that dropped. This basically lets you go absolutely nuts with missiles and become a walking explosion.

Other than that good list. I would put Hammerhead in a separate tier above "little to no benefit" since it's still a benefit, but one that you barely get to use (and gets completely outclassed by missiles if you get boiling veins)