Here comes the BUT - how can I be sure that I'm looking at the correct John Robertson (1758)? All the other trees on ancestry also point to the mother as being Isabel Smith. I just want to be certain.
Don't we all
. But you can't be certain, unless you find some independent evidence to confirm it, for example wills, gravestones, sasines, deeds and so on.
What you must never do is assume that, just because there is only one possible candidate in the records, that one is necessarily the right person, and especially when the names are as common as Smith and Robertson.
Many people's baptisms and marriages are not in the available records, either because they were never recorded to start with, or because the record has not survived. Some people estimate that 40% are missing.
In my own tree I have 3709 people born between 1800 and 1854 in Scotland. Of these there are 763 people whose approximate year of birth I have found from either the census or a post-1855 marriage or death certificate, but whose baptisms I have been unable to find. Then I have 128 people whose dates of birth I can only guess at. That means that just under one in four of the people in my tree who were born between 1800 and 1854 are missing from the readily accessible baptism records (IGI and SP). A small proportion will probably be in the records of other religious denominations, but I estimate that at least one in five baptisms is actually missing.
The further back you go, the more are missing. Between 1750 and 1799 I have about one in five baptisms missing, but from 1700 to 1749 one in three is missing.
As for trees on Ancestry - just because there are dozens of these does not mean that they are correct. What happens is that someone makes assumptions, uploads a tree based on these assumptions, and umpteen other people come along, find that tree, assume it is correct and copy it, so over time there are dozens of trees with the same assumption.
Remember what Einstein said when the book
100 Authors Against Einstein was published. "When asked about the book, Einstein retorted by saying “Why 100 authors? If I were wrong, then one would have been enough!” See
http://weeklysciencequiz.blogspot.co.uk/2013/01/a-hundred-authors-against-einstein.html