difficult
hard
questionable
a bit thick
a bit thin
absurd
arguable
at issue
baffling
bewildering
beyond belief
bothering
confounding
confusing
confutable
conjectural
contestable
controversial
controvertible
debatable
deniable
discomposing
disconcerting
dismaying
disputable
distracting
disturbing
doubtable
doubtful
dubious
dubitable
embarrassing
enigmatic
hard of belief
hard to believe
iffy
implausible
in dispute
in doubt
in dubio
in question
inconceivable
incredible
intricate
mistakable
moot
mysterious
mystifying
not deserving belief
open to doubt
open to question
open to suspicion
passing belief
ambiguous
ambivalent
conducive
creditable
ephemeral
equivocal
indecisive
intangible
objectionable
paradoxical
prejudicial
problematic
synthetical
ticklish
undecided
undefined
unexplainable
unfavourable
unsatisfactory
untrustworthy
valueless
fanatical
grammatical
mathematical
sabbatical
viatical