It is apparently playable in multiplayer. I have never tried it, but have heard that it works without any more issues than regular BtS. It is possible that there may be issues with some BUG options needing to be the same for everyone, but I'm not sure - this is a common problem for using BUG in multiplayer but it might only be an issue with options that are only available if you also have BULL incorporated into the DLL (which FFP does not), as someone who plays in multiplayer games you should probably be more familiar with this than I am.
There is, however, an "issue" that could have significant effects in a PBEM style game which everyone playing should be made aware of.
There is a problem on the first turn when loading games from saves. Since every turn is loaded from a save in a PBEM game it is always in effect for everyone on every turn.
There are intermediate values used in calculating the yield values (food, production, commerce) of a star system that are only updated when doing the "end turn" processing. These values are the direct contributions from buildings to the yields. For example, the +8 commerce from the Capitol building. The contribution is there, and will be used, when you load the save since the total for the yields of each of the 3 types are stored in the save, but the intermediate values from just the buildings are not (they are set to 0 when the game starts). However there is an effect if you move a point of population from one planet to another during the turn. This immediately causes a recalculation of the star system's total yield output for each of the 3 types but the values it uses for buildings are all 0 since the processing that happens between your turns has not happened yet so these still have their initial value of 0. The unfortunate thing is that the processing for this is only done once per turn and it is done after the star system's output has been used to add food to the current total, production to whatever it is building, and calculate the commerce values (credits, research, culture, espionage) and add them to your totals too. So if you move a population point during the first turn after loading from a save (which is every turn in a PBEM game) you do not get any direct contributions to the yield values from buildings. Buildings that adjust the yields of planets work fine, like the +1 production per point of population on a planet from the Mining Facility, but those that directly add to the yield (like the Capitol's +8 commerce, or the direct +1 production from the AstroTech UB) do not. Percentage modifiers work, but only on the yield that comes from the planets.
For many star systems other than the Capitol this will not be a problem since they may not get any direct yield contributions from buildings. For most of the rest it will usually only be a small amount of loss, some small fraction of their output. For the capital star system this can devastate your economic output, particularly in the very early game when the +8 commerce from the Capitol building represents the bulk of your commerce output. In a PBEM game that gives you a choice: when your population grows you can either stick with the location the new population point is automatically assigned to and get the +8 commerce from the Capitol, or you can move the population point to where you want it to be and lose the +8 commerce for a turn. Once you get to your happy cap and don't need to grow anymore in population you may want to shift the population around to planets with less food output but more of everything else, but in a PBEM game it will cost you in a way that it would not in a regular game.
This only affects the direct contributions to the yield values, not direct contributions to the commerce values. You should not lose the influence (AKA culture) output of buildings, or the +1 research from a Farm (built on the planet with the Grain resource).
I do not know if this affects the +1 food and +1 production per trade route that the Red Syndicate gets from their trait. There is no other trait that gives a direct change to yields. The +1 food per planet from the Utopia civic should work just fine since it is a planet yield adjustment and not a direct contribution.
It is, on the other hand, the same for everyone else too. One issue that makes it uneven is that one civilizations (AstroTech, as mentioned above) has a UB that directly adds to a yield value and this will therefore affect them more. AstroTech is already considered to be somewhat weak (their trait is generally considered to be not great and their UU is one of the two that is weaker in direct combat than the original version) and this only makes that worse.
As long as everyone playing the game knows this, it should not be too bad. You just have to resist the urge to move your population around from planet to planet except when you really need to, unless the buildings in that star system have no direct contribution to the yields.
You can actually see this effect on the first turn of the game too. The +8 commerce from the Capitol is not applied for that first turn since no turn has been run and the yield contributions from the buildings have never been calculated so it uses 0 for each.
