Downsides (imho) i see are probably the hitbox is a bit unforgiving, no checkpoint made me quite anxious, and the swim-up is a bit hard to control.
now the good sides, i really like the gameplay, the control is quite smooth, i can shoot as much as i want, having skip button is very thoughtful (i have skill issues), i like the music, the plot, the humor and sound effect, really fits what it does, explosion when the boss dies is genocidal.