Are the specs for ActiveSync publicly available? Can it be implemented without paying royalties to Microsoft on a per-copy basis?
I looked and so far have neither found the specs nor how much it would cost to get and implement them. Considering that, ActiveSync doesn't look like a suitable alternative to SyncML for open source solutions.
Technically I cannot tell either whether it has advantages over SyncML, exactly because I haven't seen the protocol definition. I suspect that as soon as a variety of less-capable devices used it, it would run into the same issues as SyncML. Poor or inconsistent support for PIM data itself is not a problem of the protocol used to exchange it.