SF3 was my first. People used to swear by SF4, as if it was better. I don't understand that choice.
It took me two tries, but I did complete a whole SF4 world, all the achievements, too (aside from broken Twilight Forest ones). It was good, but I liked it less than any other I played, including Stoneblocks and the new SkyFactory One.
I like the Nuclear Craft more than most other big energy production mods, but SF4 has nothing to spend it all on. So, yeah, I made a max size fission and then fusion reactor, which is fun, but without Draconic Evolution or something to sap it away, it feels like it ended much sooner.
That and the linearity. Though with some packs it's kinda preferable, I just guess it isn't the SkyFactory way.