It depends on how Riot feels day to day.
One minute, the Darkin are so weak that their invasion of Runeterra results in their entire race being wiped out and the last 5 being humiliated by being trapped in their weapons.
The next minute: the Darkin are the greatest threat Runeterra has even known, and will surely bring about the Apocalypse itself.
There is no consistency to the Darkin's power level, just as there is no consistency to what the Darkin actually are (Weapons or wielders? Daemons or aliens? Lore exists to support all of these contradictory positions).
We will only know how powerful the Darkin are if the lore team ever decide to create consistent lore, which they haven't yet bothered to do.
In the mean time, the power level of the Darkin will depend purely on whether Riot have a new Darkin to sell to Little Timmy.