Scythia and Sumeria just seem too powerful in a setting where you'll have close neighbours.
I don't see how to defend against a War cart rush without spamming warriors from the get go (which loses you the game against everyone else). It's 30 strenght against 20 from a warrior, and the first Heavy Cavalry promo actually gives +10 against fortified units. The sumerian player can even go for Military Tradition early, before PP, and get the +50% on War Carts + support bonus, that nobody will have. I actually think the Ziggurat is a good improvement as well, since flatland improvements in Civ 6 usually suck (and science just got harder to come by).
Meanwhile, Scythia after Horseback Riding is just too good. Healing after kills (i.e. when you most need it, since the units that actually kill are the ones that become the most exposed) and combat bonus against damaged units is bonkers (1UPT means most of your units are damaged at all times, healing takes a long while in Civ 6). Also, the Kurgan means you are very likely to get one of the early pantheons, except for a lucky opponent (faith at the start or faith CS), so you can choose GotForge. And to top it all, the double light cavalry... That's completely crazy. ideally you can spend just a few turns into the 50% cavalry policy, chop some trees, get a huge army in no time and change back to economic policies. And take into account that Cavalry are a unit with an easy beeline, so you can also get double cavalries (even though at that point it's easier to upgrade than to build new ones).
Does Scythia gets double units when gold or faith buying them? If that's the case, I can see faith-buying Cavalry as a pretty good way to finish the game, after a strong rush start.
Prehaps the more Diploey meta game that PBEMs tend to have can make player teams stop this rush juggernauts, especially in a game with more players. Perhaps I'm missing something from the expansion. Perhaps the players that got Sumeria/Scythia won't play them as I think you should, really spamming units.
And a quick pun: Scythia is not a one-trick pony Civ, it's a one-trick two-pony Civ.