I used this kind of update a lot, and never had any problem. Worked even in 11.9.2. I'm often critical of Sybase, but the update-join is one of the more nice features, which takes the set-based paradigm one step further than Oracle.
In addition to what Ted said: beware of NULL values. The '=' condition in the where clause won't be satisfied if you have nulls on both sides.
And beware of the cardinality issue. Once, by mistake, I left an extra table name in the From list of a similar Update statement. That extra table was only growing by a few rows every day. A month later, the server was running out of log space. I suspect that internally, the Update writes to the log every change it needs to carry out, even if some changes affect the same records all over again.