Burst is terrible because it's not an extra modifier. For example, SSRs have 500% Burst modifier already (or around that, I forget). So 150% from Burst Union buff gets them to 650% damage on Burst.
My Union typically puts 100 on it on day1, for Burst Hour shenanigans. Just note that this is NOT ideal at all.
I honestly don't know what you should be spending your Grails on in this one.
+100 Damage is a given every time. 100% Character attack is really good, and bridges a lot of power-gap between newer and older players.
+30 Affliction is probably by far the best after Damage. Putting 30 points here is what I'd recommend (you need about that much to guarantee your Soul's debuffs landing with elemental advantage).
+100 Affliction is the best for those who have (Awakened) Thor - and a giant middle finger to everyone else. Don't do this if your Union isn't whale central. DO THIS if your Union IS whale central.
After that... I honestly don't know this time around.
+150% Heal is really good usually, but here risky due to random Zombify. Buffing this risks your AABing Sol just wiping the team in one go.
+100% HP is really good usually, but here it'll make the DoTs hurt you more, giving you a +-0 benefit. Unless you can get the DoTs to hit cap, which we're not sure about yet. Ult Sloth Overdrives start hitting pretty hard at high levels though, so you might need HP for that.
+Double is the best damage output one after Attack, as far as we know.
+Resist doesn't make you even close to immune to debuffs, so is this worth putting points into? It does reduce the chances of Zombify happening, but does that actually help or not...?
I'm currently thinking about running 100 Attack, 100 Resist, 30 Affliction, 50 HP, 50 Heal, 70 Double. But if anyone has bright ideas, I'd be all ears.