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Perhaps, as it really depends on conditions and the length of exposure we're talking.
Some disadvantages I could think of:
1) the OM-D's battery life is pretty short compared to typical DSLRs, so that could mean running out of power.
2) Most native m43 lenses have focus by wire - so, achieving accurate infinity focus might be a problem if conditions are dark enough and you can't find a point source to focus on. Usually solved by bringing a flashlight, but the problem is there moreso than for a DSLR , though most DLSR lenses have a bit of play in infinity focus also.
Really depends on what sort of stuff you're talking about, though - astrophotography or just daytime ND-filter type stuff, etc.