"equally harmful" is correct and usable in written English.
You can use it when you want to describe two things/situations that have similar detrimental effects. For example: "The environmental damage caused by burning fossil fuels and deforestation is equally harmful for the planet."
It is worth pointing out that experienced cult experts make a significant distinction between generally harmless cults (the die-hard Boston Red Sox fans or the Yale Skull and Bones Secret Society) and dangerous cults (David Koresh's Waco group, and Jim Jones Jonestownn fanatics), and point out that all cults are not created equal nor are equally harmful.
But not all nanoparticles are considered equally harmful.
But sharia councils, which are charities, are equally harmful since their mediation differs little from arbitration.
On the other hand, too much preplanning and detailing for an interview can be equally harmful.
In Gaza, the will of the majority has proved equally harmful.
But the emphasis on the Asian approach advocated by the Kims may be equally harmful to children.
It is equally harmful to the victim whether you sell what you have stolen or give it away".
In the past, both vaping consumers and businesses have expressed concern that the conflation of vaping being equally as harmful as actual tobacco product was not only wrong, but perhaps even dangerous and harmful for those who are trying to quit.
Peer reviewers who do not engage with the manuscript and do it merely to say they review for a particular journal is equally as harmful as an author who fails to provide full details in how the study was carried out.
On the "Garden Island" of Kauai, the industry controls over 15,000 acres of prime agricultural land, which they drench with over 17 tons of restricted-use pesticides each year, and likely at least five times that amount in non-restricted pesticides that may be equally as harmful (such as glyphosate).
But no arrests have been made because every time the DEA labels a chemical compound in spice an illegal controlled substance, the manufacturers in China replace it with another compound that is equally as harmful.
