I like this game despite its chronically out-of-print referee's book, the glossy page presentation, and the feeling of "5E Mass Effect" drawing me in. The fact that the referee's book is only PDF hurts, and I wish I had a hard copy. I may order a B&W spiral-bound copy.
And Ultramodern 5 is a much more complete and expansive game for 5E sci-fi and far more popular. UM5 feels cyber-punk and magic-oriented and reminds me a lot of Shadowrun. It is an excellent 5E system for sci-fi and is well-supported.
Esper Genesis cuts closer to the base 5E rules, making it easier to mod and play with other 5E games, such as Level Up Advanced 5E. So, if I wanted to swap out the rule engine for A5E, that would be easier with EG than UM5.
Esper Genesis has fantastic classes and powers, all tech-based, and they feel very Mass Effect to me. The default setting is exciting and moldable, and you could play everything from a 5E version of Star Frontiers to anything imaginable. I can see this powering a simpler version of Starfinder quickly.
One strange drawback is the weapon tech, which assumes a default projectile-weapon universe. There are guidelines for swapping out damage types to "hack in" a laser pistol, but no laser pistols in the game. I would edit the weapon list to suit my game (and make more guns high velocity to up the damage to unarmored targets). I would add lasers, blasters, and other sci-fi favorites.
Laser pistol: 1d6 radiant damage, 60/300 range, 2 lbs., high velocity, 20 shots, 500 cr. If you wanted the Star Frontiers 'varying power levels' just give it three settings: 1d6, 2d6, and 3d6 damage. Make ammo consumption: 1, 3, and 5 shots to account for diminishing returns. Make a natural 1 an overheating result for the two higher power levels (like a jam), and the weapon needs to cool down for a combat turn (or two for setting 3) before it is used again.
Game design is fun! Play games that let you do this.
There is also that thing in sci-fi RPGs of "too many weapons, who cares, only use one." We played the original Space Opera and had characters who only had a blaster pistol and never used another weapon.
I found it strange the weapons did less damage than the ones in the DMG. Still, they have weapon properties that double damage targets with no natural armor (this term is crucial once you open the monster book), so they try to balance lethality with playability. Less damage is a more intelligent choice, especially with a party of four all blasting away. As usual, the warriors get action surges and extra attacks, so DPS should scale along 5E levels.
I would rather have the weapons doing 1d6 to 1d12 damage instead of the 3d6 sci-fi weapon damage in the old DMG and keep turn damage predictable along 5E levels; otherwise, that party of four is doing 12d6 damage a turn (plus ability modifiers) if they all hit and will vaporize anything in front of them below ten hit dice. Tell yourself, "It is all cinematic damage," and it will be fine.
EG is a game that deserves love, attention, and a second edition. What we have today is impressive and a solid sci-fi game that presents the basics and does not try to do too much. UM5 does everything, and I find that book easy to get lost in. EG does a Mass Effect-style game perfectly, and it has room to mod in plenty of custom ideas.