Of course they are, the trick is to get the most utility out of each one. I work at Tokutek where we use a data structure that does this, in the sense that when your working set is larger than RAM, we still don't incur very many I/Os for writes. If you want durability, there's nothing you can do about the logging fsyncs except buy yourself a nice battery-backed disk controller.

Partitioning is often a bad idea because it messes with your queries. I don't know what's new about partitioning in InnoDB but I think it's generally a symptom of the over-use of B-trees, which don't try to do anything smart about random writes. The change buffer is a decent idea but it's just a stopgap, when you have enough data it doesn't make a dent any more. A better idea is to use a data structure that can handle lots of writes.

I have just started learning about Riak, and from what I understand, they need to do a query (so, a disk seek) on every insert (to calculate something with vector clocks), so they aren't actually using the write optimization that LevelDB's LSM-trees can provide. I don't actually think it should provide that fantastic performance, but I should admit I haven't run it yet. Maybe they're more interested in the compression LevelDB gives them.

Shameless plug time! http://www.tokutek.com/2011/09/write-optimization-myths-comp...