Couldn't speak for 6mm, I'm afraid.
For 28mm, the most complete range is the old Perry's range they sculpted for Foundry back in the 1980's. Even here, their are minor inaccuracies and some gaps: no horse artillery and next to no mounted officers or staff, just the two vignettes of colourful staff lounging around tables. This range was done back in the days when you bought individual figures, and when Foundry forced everything into packs, circa 2001, they messed up the Prussian infantry by including some figures in greatcoats within every pack. That might work for WWII maybe, but by rights an entire unit should be wearing their greatcoats or none at all. You can make tolerable units if you allow for this, or aren't bothered!
The other failing is that the artillery pieces are poor. The information just wasn't available back then, I guess. For the French it's not too bad. There's a tolerable 4pdr and mitrailleuse, just no 12 pdr, which formed part of the corps artillery. The piece in question was basically a larger muzzle-loader with a (semi) block trail, so there's things that would pass muster to fill the gap. For example you can buy individual ACW gun sprues on the current Perry's site, for just £3. The picture is more difficult for the Prussians. The Foundry range has a "Prussian Field Gun", presumably meant to be the 4pdr C64/67; it's a pretty generic model, wrong in several ways. They also do a somewhat larger "Krupp 90mm Gun", which looks a bit more like the 4pdr C64/67, but is actually based on the 90mm Model 1873. When it comes to 6pdr's which comprised 50% of the foot artillery, there's nothing at all, and I don't know what to suggest.
The next most useful range would be the North Star 1866 Prussian range. It's not complete, but the infantry and Uhlans are very nice and we are promised some more cavalry at least in 2018. There were supposed to be minor uniform changes since 1866, but the old versions were still being worn, so no problem there.
Finally an 1870 range was started by Bill Thornhill/ Musketeer/Footsore, but only got as far as French line infantry. There's several packs from which decent units could be assembled. I think the proportions are odd in some areas, but these are nice "modern" figures, which would paint up a treat. The range now seems only to be available from a US company called "Wee Wolf".
Hope this is some help. FPW is a colourful and dramatic period, crying out for a full modern range, but we have to make do for now!