Within a genre, people will criticize games that have too much inertia. If considered from a 1p shmup perspective, Senko would look like it has a lot of inertia. This would be a bad idea because it is not a 1p shmup any more than it is, say, a 2p puzzler. Within the 2p fighter realm, as you say yourself, it is at the bottom end of the inertia spectrum. This is why complaining about the inertia is stupid.
By saying that inertia is a "detrimental factor," you're implying the lower the better. This is not true because no fighter has zero inertia, because it has a purpose: to allow punishing. There is an optimal range, and while Senko doesn't fall in the 1p shmup optimal range, it is actually on the lower end of the 2p fighter optimal range.
I can find problems with any game I like (there isn't even a single developer with >2 games to their credit that I really like every one of their games). It's just that in Senko, inertia isn't one of them. The biggest problem I see is that being a boss isn't interactive enough. Attacks should be more controllable and dodging more possible, and to balance, make them less buff. (Ironically, if you said the boss aspect of Senko has too much inertia, I could agree with you there, but that's not what you meant.) Bosses are also poorly balanced at the edges of skill: they're too strong newb vs newb, and too weak pro vs pro. These would be addressed if the bosses were more regular-sized; tweaking the large boss formula wouldn't solve it.
As for the non-core stuff, this game has a steep learning curve but the game itself provides you almost no support for learning, not even any text explanation. Story mode alternate endings are obtuse to trigger and don't give feedback. Score attack is highly luck-based to get top scores and based on checkpoint milking. Live is implemented poorly, as cheating and piggybacking are rampant and make rankings mostly pointless, and there aren't options for you to find similarly matched opponents anyway. The font is overly stylized, and in particular, it is extremely difficult telling apart the 1 and 2 digits (highly important!) without significant experience.
BTW, you are the fanboy.
