1. Fluff wise, most games don't take place on Earth, or even in our Universe. This would take more legwork from a DM if a player wanted to play it because it would need to be adapted.
2. A lot of the grammar and writing is confusing and could be cleaned up. For instance, what is 'proficiency in Wisdom rolls' from Neptune? Proficiency in Wisdom ability checks? Saving Throws? Spell Attack Rolls?
3. Flat bonuses (especially to AC) are bad. IE, a Warforged Druid at level 14 could easily hit a static AC of 26 while in Big Bang, assuming no multiclassing and a Dex 14. Plus it gets a free shield to boost up to 31 when needed.
4. Really broken multiclass potential. Its not something you're supposed to worry about too much if the class is otherwise solid, but a Paladin (or Hexblade) will bust this wide open. Elliptical orbit and such isn't so bad on a chassis that's normally mediocre at melee combat, but if you drop two levels of Paladin it gets crazy. Resistance to BPS, Increased Crit Range (Smite), and Bonus AC are all particularly problematic.