In the Modern does White have all that many options to prepare? The opening seems rather limited for both sides to me, at least after 1.e4, 2.d4 and 3.Nc3 which will be most common. So Black can probably rely on being more familiar and more experienced with the typical positions and the sharper main lines.
Personally, back when I always played 1...f5 vs 1.d4/c4/Nf3, I had a few instances of serendipity where someone brought out something prepared and I had previously faced that line in online blitz and looked it up. In one case, a friend of mine who also played the Dutch asked me about a certain tricky line in the Staunton, I looked it up for him and just a couple of days later I faced that same line in the deciding Armageddon game for the club's rapid championship, and won easily despite only needing a draw.
So, in my humble opinion it's quite possible to play the Modern all the time, at least on a non-professional level. Occasionally, one could perhaps mix it up with a Pirc or Gurgenidze - on the amateur level you usually know exactly which opponent is likely to be packing heat...
Though, it may well be too tempting to find out what they've prepared!
As long as you're really interested in the opening, keep up with top level practice and make sure you're well booked up against the three big ones, Nf3+Be3, f3+Be3 and f4, (plus recurring minor mainlines & tricky sidelines).
Most of that will come from "being really interested" as mentioned, and even with computers, most players won't be interested in spending the effort to find something new deep in the Modern maze.
So theoretically Black should be okay - but you will of course also need to play well, as most games are decided on mistakes and those are usually made by the underdog...
Btw, if someone has the understanding and energy to prepare something new and really strong against the Modern, then it's likely that he/she would've beaten you in any other opening as well. On the plus side, some of your losses could actually turn out to be surprise weapons for yourself when facing your own opening.