I'll start by saying that I've only played 1900 once, and as it happens I drew Austria. I managed to solo the game, but I found that on the road to a solo Austria is more handicapped than it may even be in the standard map, and I'd be curious to hear the opinion of others on this who have played more than I have. Here's why:
The REM rule and Siberia
According to the creator's notes, the REM rule was created to reign in Germany, which turns out to be too strong in this variant (even with the REM rule). However, it really ends up hurting Austria, and here's why:
1) Even in the standard map, invading Russia from the south is pretty hard beyond Sevastopol
2) With the REM rule, even if you invade Russia and say destroy one of its units, it gets to be rebuilt
3) With the addition of Siberia, now it's even harder to invade/hold onto Sev
4) With the addition of Siberia, picking up Moscow from the south is now even harder
Of course, someone invading Russia from the north (say Germany) only has to deal with the REM. StP is a little less vulnerable thanks to Siberia, but still is bound to fall. I am wondering if the game would be more balanced if the REM rule only applied to Germany, as after all that's what it is designed for.
Germany and Italy are significantly stronger
This is pretty much self-explanatory. Judging from the map's game stats though, it appears that 1900 has made central triples more viable, so I'd be curious to hear the point of view of someone who has actually tried that.
Greece is impenetrable without fleets
Greece has become pretty much a fortress in this variant when one attacks from the north. The only way to get it is having at least one Ionian fleet. With that being said, now there are two areas adjacent to Con which means it's easier to grab it by land, so perhaps that balances out the Greece fortress.
These are the three main points, but I think the Russian point is really strong. I also found the addition of Bosnia somewhat unnecessary, but perhaps that's just because I didn't have to defend my homeland.
Advantages to Austria
I do of course also see some advantages to Austria:
1) ATs are much more viable now, as Turkey has an African path, so it doesn't have to roll over Austria to grow
2) Thanks to Switzerland, a successful Austria in the mid/late game can grab an additional center before getting bogged down at Munich
3) Austria now has more of an advanced notice before an Italian stab happens thanks to Venice not being a home center (well known)
4) The other well known advantages regarding Galicia, and Austria's 3 starting armies
5) Austria can potentially grab some African centers, although tough given its single port and no fleets to start with
Despite the above points, I think that Austria is even weaker in this variant than it is in vanilla thanks mostly to the changes made to Russia, and its relative solo rates seem to confirm that. But I'd be curious to hear the opinion of others who have played it more than I have.