I think the OGS rating system is not like a crapshoot, but OGS ratings are more volatile than ratings on other servers. There are several topics about it (like this one).
As far as I understand, this volatility is by design. An upside is a quick settling towards a roughly correct rating for newly registered players. A downside is that ratings of established frequent players may fluctuate by a couple of ranks, while those may only fluctuate by like half a rank on other servers.