I have had limited luck with smaller cM matches (down to single digits), BUT I also know the names of the majority of my 2G grandparents (and some going back much farther), so that makes a huge difference because I can check matches against other posted trees (many people do not have trees on the site). If you see someone with a private tree, do reach out to them, provide some information about your family, and ask if there is any match that they can confirm. Even if they do not have a posted tree, reach out to them, because they are probably having the same issues as you, and I have found a lot genealogy is collaborative work. (I have some strong matches who have an adoption in their line where the connection is probable, and I circle around with them occasionally to find out if they have discovered anything.)
Also, ask older relatives what they know about their parents, aunts, uncles, and cousins. Even if they were born in the old country, there may be some names that may provide clues for you.
Generally, I find anything below a 3rd cousin match to be the "easy" cut-off point (G grandparents were siblings). But there are some techniques you can use to confirm more distant relations.
In Ancestry, I would always select the person, then check the "Common Matches" tab and look for those common matches that you are able to confirm. Granted, this does not always work because I have experienced individuals with common matches on both my maternal and paternal lines, so I cannot narrow down the connection(s).
And, if you have a tree on Ancestry - public or private - under the "DNA" tab there is a newer item called "ThruLines." This is a quick way to check against other people's posted trees. I have had success using this to connect with - and share information with - cousins descended from my 4G grandparents.
The more you know (or can find out) about your own ancestors and the names of the siblings of those ancestors, then the better your chances of success. So don't be discouraged and give up.