They are loud when they recharge, they are slow compared to nuclear subs and they carry much less armaments.
On the other hand, when they are on battery power modern diesels can be much quieter than nuclear subs, they are much cheaper and smaller so ideal for operations in coastal waters. Which is why many (also western countries) rely on them for coastal defense.
Economics wise: You can trade 3 diesel subs against a nuclear subs or a large warship and still come out ahead cost/effort wise.