That’s if their API implementation itself is just bad and underlying DBs aren’t . If they or someone else with bad practices manages the DB you may be in worse situation than before. In general, to me, shared DB is bad because it is hard to not cut corners in such design and ensure that DB owner does not break contract for all consumers. This is basically why APIs created - to guarantee contracts and encapsulate change. But I digress. My point is that it will be your responsibility to ensure schema changes adopted to expected contract. If data is not normalized/structured, like say, it’s JSON then I would stay away.

Does API actually doing anything or it’s pass-through to data source? You could request data feed and build adequate API on your side