Nukes are not only expensive and hard to construct, but the special horror about them makes them most effective when not used, as a sheer deterrent. (One might argue that they’re what kept an outright third world war from breaking during the 20th century—we might have come close to the brink, but we stilldidn’t pull the trigger.)
Biological weapons are self-replicating. It means when you prepare some special virus and spread it across the small group of people it can continue spreading up to size of global pandemic of the whole world.
Biological weapons can be used also against some species of plants and animals and so destroy food sources of enemy.
Impact of biological weapons is today hardly predictable – how the virus will be spreading.
Bio weapons, also hard (but not nuclear-impossible) to properly create and stockpile, also have a terror value to them, again albeit not as strong as the world-ending variety of nukes.
The biggest difference is that the biological weapon doesn’t destroy the infrastructure, and in some cases can be targeted to kill people only.
And the biggest advantage of Bio Weapons over Nukes are they are (If done correctly) virtually impossible to notice until it kills millions, one could virtually spread a canister filled with viruses ( thought it would be a suicidal mission unless it was set to timed release) designed in such a way that they remain dormant until say it being exposed to extreme cold and humidity namely the winter season, So until then it enters and infects thousands and keeps on spreading and as the winter starts boom people start to drop dead so if done.
In conclusion, while biological warfare has more potential, it has more risk of backfiring than a nuclear bomb.