hiding the dropbox cache folder from spotlight as posted earlier wasn't the solution... mdworker came back 😞
What worked temporarely for me was deleting the hidden .Spotlight-V100 folder on the root of my systemdisk (where dropbox stores its files as well). After a restart it took some time to rebuild the entire spotlight index, but mdworker didn't show up again and smart sync and searching in the dropbox worked like a charm... for about one day, then the nightmare was back.
So far: none of the many "solutions" posted here and on other forums worked. Since many many months now. Please, Dropbox, stop ignoring your customers, please?!?
I unlinked and relinked my dropbox and for a while it reduced the problem, but with the latest High Sierra update it's back with a vengeance. I thought smart sync was a blessing but it's turned into a curse. I'll have to seriously consider dropping dropbox (I've been a paying customer for years) as this is literally stopping me from working and has taken hours to fail to solve. Plus it's seriously irritating.
I know you are at the mercy of OS updates, like we all are to Google algorythms but please keep up and on top of these things.