You have singled out Ancestry here, however once you start getting 'knee deep' in your research, you will find that there are many more examples where information may not be as accurate as you would like, be it 'user sourced' or official documentation!
Birth/Marriage/Death records may not contain factual information and ancestors have either adjusted the truth to fit their lives (IE listing the wrong father on marriage cert, or typically wrong age...) or simply didnt know/guessed. It is why its important to obtain as much documentation as possible to verify things like age, and links between individuals
I always take other peoples work with a pinch of salt and always verify. Its great when you find a tree with information in that can help you, IE maybe you have missed a sibling, so looking at other tree's may help help direct you, but should never be trusted as 'the truth'.
Other websites such as Findagrave/Billion Graves - I only ever submit an entry which I have taken a photo of a headstone and then submit exactly what I see on the stone, adding nothing else, however there are others who attempt to be helpful but adding extra information that may not be correct which can be difficult to get corrected.
FreeBMD - again, I submit Postem's with what I see on a certificate and nothing more even if I know more, yet some others submit more information and there are no checks or ways to edit a previous entry.
Newspaper articles are very helpful, but sometimes contain inaccuracies.
Your 'job' as such is to take this information found and fact check it... is this the right record and is the information on it accurate, and does it fit? It maybe a case that it is the correct record, but a marriage may have the wrong father listed.... which is when you need to ask 'why is this'. I had this and ended up unravelling a whole different family structure and narrative because of that one record.
There will be times where you end up thinking you have the right person, only to find out you have been barking up the wrong path.... it happens!
Ancestry and other commercial sites are in the business of selling subscriptions and services so they will always want to make it appeal to a wide range of people. We live in a world where we want everything now and as a product of that, Ancestry have tried to help this market with their tools. These tools used correctly can be a massive time saver, but used incorrectly will result in an inaccurate tree.
Family research is difficult and time consuming but can be very rewarding. There are no shortcuts and a single person can take hours, days, weeks or even years to complete. Set yourself goals and limits to make your research manageable (IE my rule is siblings of direct ancestor + marriage + children.... then stop unless there is something of particular interest), and dont rush yourself to go back as early as you can as attractive and fun as it sounds to work your way back to 1600/1500/1400 or earlier and verify each generation as best as you can before progressing onto the next generation back. Seems to have worked well for me but you will find a method that works for you.
