After searching the corpus (millions of English texts), "considered as" (3,000) is significantly less used when compared to just "considered" (108,000). 3,000 uses is not insignificant, so it is difficult to say that it is necessarily "incorrect".
I found some theoretical grammar difference, but it is largely not observed and writers often seem to use "considered as" as just another alternative to "considered".
Considered + object
Used when the subject should be just another name for something. Literally, that the object is the subject.
He should be considered our leader since he makes all the decisions.
= He is our leader.
Considered + as + object
Used when the object can be used to replace the subject.
Indoor soccer should be considered as an alternative to outdoor soccer in winter.
= The two are equally good