I have the same problems - both with spelling and with words it doesn't recognize. To be honest, I sometimes just go with something I'm pretty sure is misspelled at the time and just have to look it up later to correct myself.
I know you can erase your personal "dictionary" in it completely through the settings, but it's probably easier just to check yourself with another spellchecker or a dictionary when you can and then correct Swiftkey's prediction's.