In PATB-tokenized MSA, the corresponding type of tag set (core POS extended with a complete description of morphology) would contain upwards of 2,000 tags, many of which are extremely rare (in our training corpus of about 300,000 words, we encounter only 430 POS tags with complete morphology).
researchers have proposed tag sets for MSA whose size is similar to that of the English PTB tag set, as this has proven to be a useful size computationally.