I've been following this thread with interest and some bemusement. Whether the online genealogy sites are useful depends a lot on what you're trying to get from them. Those that post images of original documents are of clear use since the best anyone can do is find those original documents. Those that use OCR to "read" documents and then post what has been 'read," are also useful, as long as you understand the limits of OCR technology.
Family trees posted to any online site must be taken with a grain of salt since there is essentially no way to know the skill level of the person posting it. But we also need to recognize that skill level and accuracy don't mean the same thing: a broken clock tells the right time twice a day. I have used these posted trees to great positive effect -- they can be very useful for identifying people who might otherwise be unidentifiable -- but make a point of back-checking anything I find in one. I don't expect the site to back-check data, but that doesn't mean I shouldn't do so. Use of these trees has enabled me to find half a dozen previously lost "trunks" of my various family trees.
I started using Geni many years ago, but quickly realized it was a kind of tar-baby, once you got involved there, you didn't know what would happen. Geni has sent me about a dozen "matches" over the years. They are of two types: people I already knew about, but hadn't listed with them, and people who couldn't possibly be related to me for various reasons, but who had some connection to a person who shared a name in my tree. No competent person would make such errors, but a computer algorithm doesn't know any better.
Conclusion: Know what each site is good for and use it for that, only. If there are sites that are good for nothing, don't go back to them.
Researching: ZUSSMAN, ZISSELMAN, ZUSSELMAN, CRISS, KARDONSKY, GERROL, KREMER, AINGORN, all Ukraine, possibly plus USA
AKABAS, MALACH, YELLEN, SHAPIRA, SWIG, GRODZENSKY, SALINGER, ZLOTNITSKY, EISENDORFF, all Lithuania, possibly plus USA
FIZYK, THORNER, both Poland, possibly plus USA