Sorry for taking a while to reply, multiple reasons I haven't been able to.
It doesn't have the extreme versatility of Dominion, obviously. Instead the replay value is provided by two things: larger variance and player interaction.
Let's face it, in Dominion there are two possible starting game states. On many boards, no matter the variety of cards available, there are only one or two viable openings.
In EmDo, there are a large variety of starting states, and your starting hand and planet will start to determine your initial strategy, with your first Survey likely forcing you to choose a direction. Other players' role choices will determine a lot of what you can and can't do, and in many cases what you should and should not do; this is similar in scope to Puerto Rico, but without the fully-open game state of PR.
As to the other question: undoubtedly you must specialize in one planet-taking method or another... if you're going to base your win on planet points. If you're going for a Research or Trade strategy, you can often get away with just three to five planets. In that case, you may be able to hybridize your planet-acquisition... I'll have to try it sometime!