No significant difference in surface does not equate easier to dominate slams. There will be more good players to adopt their games to be better on the surface. Sampras had a hard time winning on clay because his game, which was perfect for Wimbledon, has many holes for RG. He tried to adjust his games to better on clay but it affected him on glass. Vice versa for clay specialists who did not even bother to show up in Wimbledon. You have a set of players who were great on Wimbledon, another set on Roland Garros. Even for the faster hard court for USO and the slower for AO, there are more overlap of players but still, some stand out more.
Now on the medium pace hard courts, all the top 4 players are actually very good on it. Even the rest of the top 10 can play very well. Thus, it's not that easy to dominate.
How many players in modern era managed to win all slams? And how many managed to win all slams more than once?
It does make it easier to dominate slams because everyone will play the exact same game on each surface meaning the top players will always win them. The differences are minor but they are mostly movement orientated for example Davydenko's horrendous movement on grass.
What it makes harder is for a surface specialist who excels on one particular surface i.e. a grass court expert or clay court dirtballer, to snatch his specialist slam title, or in general for a Thomas Johansson-style shock to happen. One of the key reasons for the ATP/Slams homogenising courts was cause they didn't want Johansson-type situations again. Not glamorous enough for them to make $$$$$$.