Pioneers after the Civil War often leave more direct records of their experience
County histories began being published in the late 1800s
Newspapers grew in every town and often recounted early settlement in news
or in obits etc.
Vital records are more common in the latter part of the 1800s
More people were literate and left letters or notes on the family
Those experiences in the late 19th century / early 20th century are still in many
families memory
By contrast the records that help most in early pioneering eras of the late 18th century or early 19th century are more apt to be:
Land Records - patents - deeds
Probate
Tax records
Recollections of much later descendants
History/How the culture worked
Geography
Early research is messier, demands more collecting, more studying, more everything which is why so many family trees online are reasonably accurate till this period. It is labor intensive and you have to learn new skills.
Geography & History:
history and an understanding of how families lived in earlier times provide many advantages that are lost in later eras.
Historical events are often record; militia actions, epidemics, settlements, historical events are recorded and had direct effect on your family because when there are only a few hundred people in the whole area you are a part of the history being made.
People were (mostly) predictable - and if they aren't that is a red flag to find out why not. They come from the same places, travel the same paths etc. There were few roads and navigable rivers so you had to take certain "funnels" to get to new places. Good maps are crucial. Know the place you are researching in.
In days when there was no railroad or easy transportation families stayed clustered together; they were more tribal; they migrated together. They stuck with their own group. People rarely strike out on their own. If there are no people around them with the same surname it probably means they have cousins, sisters, aunts etc. and you simply don't know they are kinfolk at this point.
Most earlier pioneers were from rural areas or very small towns Their prior communities provide clues to relationships. Land bordered relatives; certain families marry again and again. Some continue on with them. If you think you have discovered where they came from you often have to work back and forth to discover their world in both places till you figure out who matters and what they were doing.
The early frontier - you might think that families who had braved immigration to the new world and first settled on the Eastern seaboard might have stayed put rather than start the process all over again but there were plenty of reasons to want to move westward into places like Ohio, Kentucky, Tennessee and the midwest.
Most common reasons to migrate in early America:
new land / population explosion - families were large and you needed a better farm to feed them. In the places where crops like tobacco had been grown the soil was giving out. New land with better soil or lands where there were many natural resources to help you survive called people to new places.
generations carved up original farms; Grandpa got a patent for 160 acres. It was divided between his ten children who kept dividing their portions.
overcrowding - it may not look like a lot of people to our modern eyes but to those who had first settled in an area when there were minimal people the influx of new immigrants and natural population growth made them long for their more rural past.
economic opportunities - while most people put "farmer" as there occupation they often had a second income providing services to these new communities or gathering goods and finding ways to get them to larger population areas.
religion - you went with your group to form a church community in a new place. Church groups are complex because they usually had kinship ties. Some held extreme beliefs; some more standard but there was a sense of building their own community that drove them to a new place.
in trouble - family trouble; money trouble; legal trouble - you went somewhere else and started over.
When reasearching early pioneers:
Switch mode from modern thinking to "tribal thinking" Learn to ask "who was important to them?"
study the family unit - no relative is to minor to learn about. Don't be afraid to collect on people you think "may be related." It usually pays off.
the neighborhood is everything - pay attention to neighbors or any names seen with your family like inlaws, witnesses to documents etc.
Play the odds - "most people" who settled in this county from Virginia seem to come from Bedford Co. VA - check to see if yours did to. "Most people" came via the Ohio River.
Come up with probable theories and try to prove them.