"unwelcome effects" is a correct and usable phrase in written English.
You can use it to describe the undesired result of something. For example, "The new policy has some unwelcome effects, such as increased costs and fewer options for employees."
In a global world, events in China could have some unwelcome effects on the eurozone.
Plans for redeveloping the City of Newport are having unwelcome effects on Mansion Hill.
Mr. Newhouse next moves from principle to practice, cataloging the unwelcome effects of Mr. Bush's doctrinal excesses.
And that is also why some of the ancillary, unwelcome effects of #OscarsSoWhite sit so uncomfortably on the tail of this best picture win.
However, there are several issues of concern in this sector, including high rates of foreign ownership and control, unwelcome effects on the environment, rapid rates of extraction that may exhaust the reserves, and the widespread but not universal neglect of simple preshipment processing in Australia.
New mobility services have the potential to complement public transit, but they could also lead to worsening traffic congestion, more vehicle accidents, additional air pollution and other unwelcome effects if not managed carefully.
One unwelcome effect may be to raise the unpleasant profile of the kind of loan company that can successfully seal the poverty trap for good.
This story may have the unwelcome effect of making children housefly-friendly, but enjoy: it has the wittiest ending – begging the reader not to slam the book shut and make its hero history.
Might the unwelcome effect of such a film not be to turn the site of the Berlin bunker into a type of unhealthy tourist attraction, even for some a pilgrimage site?
This has the unwelcome effect of introducing some kind of need for judgement, with more than a whiff of subjectivity, since someone has to decide what is the "elsewhere" that has been studied.
He believed that filling the output gap through deficit spending was important, but that a package that was too large could potentially shift fears from the current crisis to the long-term budget deficit, which would have an unwelcome effect on the bond market.
