You can also apply cursed flames in different (and better) ways.
Anyways, just to note something
While I do think there was a contradiction, I also want to note that these two weapons are obtained in different stages so they should be judged from how they perform at their stage. Stellar Tune is a good crowd control weapon and overall performs very well for the post golem stage. Solar Eruption is a pillar weapon and IMHO it doesn't have good enough performance for a pillar weapon. Its a good crowd control weapon, sure, but for the remaining pillars, its actually not as good as some people might expect.
For the Stardust pillar, it does very well, but Nebula and Vortex? Consider how much of a hell Nebula and Vortex pillar enemies are, you don't want to use a mid ranged, inconsistent dps weapon against them. Overall, wall pierce is probably the best and most notable niche Solar Eruption has.
The reason why I think Stellar Tune should be in the same rank as Nightglow is because both of them are good weapons with different niches that balance each other. Solar Eruption (along with Nebula Arcanum) IMO is B tier because it really isn't good enough for a pillar stage weapon. Even at clearing remaining pillars (which is the main purpose of the weapon) you might do better with a weapon from earlier stages due to them having longer range and being more consistent.