Both the overall structure and the description are formalized to an extent that can orient the professional linguist, but makes the description essentially unusable for the layman.
In particular, the formulas may be hard to decipher.
Treatment of the same subject matter in two complementary perspectives leads to a high degree of redundancy.
Advantages:
The two perspectives, i.e. the functional and structural approach, correspond to the ways that users actually access the information provided by a grammar and a lexicon.
At the same time, this binary subdivision allows each of the two descriptions to be systematic and internally consistent.
Grammatical categories which the typologist may want to check for (e.g. classifiers, copula, presentational formatives ...):
the onomasiological description mentions all of them, whether or not they occur in the language
the semasiological grammar describes those that do occur.