I've owned this album since it came out in 2014, and my backlog is such that I've finally started listening to it. First, Gunstar Heroes is an album that I regularly listen to. When I open foobar and grab music, I tend to pick Gunstar Heroes a lot. It wasn't always this way. I neglected it for many years and someone requested a CD-R within the past few years, so I "rediscovered" it.
Last night, I listened to the Gunstar Heroes OST for the first time. I was very entertained, which was due to my extensive exposure to the arrange album.
The other soundtrack of significance on this album is Alien Soldier. I haven't yet listened to it yet, but I own Alien Soldier and I'm looking forward to making a comparison.

And where's Vol. 2?

I've always found this volume to suffer an identity crisis of sorts, perhaps due to poor marketing or lack of focus on advertised material. This is implied to be a representation of varied era music that was able to be licensed for release. This probably should have just been a tighter SEGA music collection and then separately a Treasure collection.

Speaking of follow ups, I'm still waiting to hear of an Evangelion Pianoforte 2 (as Shiro Sagisu once hinted another was to come).

From what I understand, the Mega Drive was just not that popular in Japan. So maybe they had trouble putting together a large selection of recognizable soundtracks for a Japanese audience. I'm just happy those discs are filled...It's not like you're getting four CD's with 35 minutes of music apiece.