I'm not mentioning tier 4 as I don't use anything but INC anymore.
The difference between euphoria/BoP comes down to movement requirements, and adds. Lots of movement + many adds with high health = BoP. Little to no movement + low health or no adds = euphoria. That's at least my general understanding of their usage.
It seems the equation for Stellar Flare being viable would be high to extreme movement requirements + low number of adds with lots of health. If that's true then Hans & Franz and maybe The Iron Maidens would be the right time to use stellar flare.
Gruul - Euphoria -- in general moderate to low movement no adds
Oregorger - Euphoria -- phase 1 is very little movement no adds
Blast Furnace - BoP -- high movement and tons of adds everywhere at all times
Hans & Franz - Stellar Flare -- extremely high movement and only two total targets throughout the entire fight.
Flamebender Ka'graz - BoP -- moderate movement requirements, the dogs are up for a pretty long period of time and need to die simultaneously.
Kromag - Eurphoria -- Moderate movement, only adds are low health hands where you wont have time to dot them.
Beastlord Darmac - Euphoria/BoP -- Unsure on this one moderate movement, lots of adds but mostly particularly low health.
Operator Thogar - Euphoria -- Lots of standing still and flat dps on boss. Lots of adds but all very low health and not worth moonfiring.
The Iron Maidens - Euphoria/Stella Flare -- moderate movement, 3 constant targets with lots of health. I think Stellar Flare COULD be viable here.
Blackhand - Euphoria/Stellar Flare -- same as above moderate movement, 2 constant targets with lots of health (tank and Blackhand)
This is all based on what I've read and seen, so obviously there's room for improvement here. The post regarding tier 7 talents for HM helped me a lot as I was just starting to learn boomkin, played resto for years.