i Register
In some senses, unbelievable is marked as figuratively, literally. Watch for register when choosing this word.
VERBS
be, seem, sound | become
find sth
It's hard to find his behavior at the meeting unbelievable given his past actions.
ADV
really | completely, quite, totally | just, simply | almost | pretty
PREP
to
It seems completely unbelievable to anyone who knows how hard she worked for that promotion.