Are these miners effective enough? I guess, at scale they should have some value but my initial gut feeling would lead me to believe that even a huge botnet can hardly compete with dedicated hardware.
Some cryptocurrency algorithms are designed to be less amenable to acceleration with special hardware, so that CPU mining remains effective. Monero, the one involved in this case, appears to be one such.
They are effective enough at creating a poor user experience, eating all the users CPU without their permission in exchange for government provided text and wasting energy.
Whoever setting a cryptominer like that doesn't have to pay dedicated hardware or the electric bill, and as you know nothing can beat gratis-free.