In real life, electrolysis and hydrogen generators is not a way to make electricity. The hydrogen generators generate less electricity than needed to do the electrolysis in the first place. But there's an important difference between electricity generation, and making an easily portable fuel source for cars, aircraft, spaceships, mobile homes / RVs etc. Liquid hydrogen can be great as a portable fuel source. So can methane (LNG), petrol / gasoline, kerosene, and rocket fuel.
In Galacticraft, the (coming soon) Water Electrolyser in Galacticraft will be part of the process for manufacturing rocket fuel on Mars out of nothing more than water + carbon dioxide. We think that once a player has reached Mars and the Asteroids, you don't really want to be wasting time running around on the Overworld looking for oil. As a great by-product, the Water Electrolyser will also make oxygen, so nothing is wasted.
Solar panels are a good energy source on the Overworld, Moon and space stations - very good on space stations - so long as you have enough space to make a big array. Solar panels are not so good further away from the Sun.... I agree with you, solar panels will be excellent on Mercury....
So yes, we need a third type of generator for use further out in the cold depths of space. It's not going to be using diesel or rocket fuel though. Burning those types of fuel doesn't work without oxygen, and oxygen is a precious resource out in space. It might have to be something nuclear.
We do assume that most Galacticraft players use Galacticraft alongside other mods, for example one of the power mods like IC2, Mekanism or Thermal Expansion - maybe fancier mods with advanced nuclear power - so most players should be able to find some way to meet their energy needs.