Ancestry is a very popular site for family history researchers. People put their family trees on there and other people copy chunks into their own trees thinking it will save them time and effort. A word of warning. Don't. My daughter recently joined Ancestry and had a DNA test, resulting in several hundred matches of varying degree. She has got me going through those from my side of the family while she concentrates on her dad's side, about which we know little.
I have been doing my own family research for about 25 years. I started with a tree of one part of the family compiled by someone else, checking his findings with original records through the local LDS (Mormon) Family History Centre scanned records. I found a big mistake. One of my supposed direct ancestors had actually died at the age of 3! So believe nothing if you haven't checked the research.
Looking at the trees on Ancestry I am finding so many errors, some minor, some huge. I can often tell what has been copied from which tree. I would swear that some of the earlier trees contain little more than guesswork. Find somebody in the online records with the same name and it must be the right person! I won't say I am perfect. Double checking has led me to correct minor mistakes of my own - but it can take me hours of research to confirm. or reject.
Also, one of my "found" cousins asked me how private is Private on Ancestry. Not very, it seems. You can hide the details of living people by marking them Living. I think this may no longer apply if they die and you enter a death date. Having a Private tree doesn't seem to be very private. If you have a DNA match there seems to be no barrier.
Another point to be aware of if you use Ancestry is that once on there, your tree cannot be deleted in its entirety. The best you can do is remove the details of every individual on it, individually.
Some sites have links to other site's records, which they don't themselves hold. Genes Connected will send you through to Findmypast for censuses but then you must pay FMP for access - but see below - you might not need to check the actual record for what you want.
In general, it is best to use more than one source for information. Different sites have different benefits. I have found Findmypast is the best for UK newspaper archives, for instance. Genes Connected (a development from Friends Reunited) lets you see a lot of basic info for free. This can save you money. They, like some other sites, allow PAYG so you "buy" access to a certain number of records without paying a monthly subscription. Very useful if you don't have a lot of lookups.