unsound(WEAK)
An unstable person suffers from sudden and extreme changes in their mental and emotional state: （情绪）反复无常的，不稳定的
emotionally unstable情绪上反复无常

SMART Vocabulary: related words and phrases

Unsafe and insecure
appealingly
at stakeidiom
be on the lineidiom
day
indefensible
insecure
insecurely
insecurity
instable
risk
security breach
shaky
sketchy
someone's/something's days are numberedidiom
threat
unsafe
unscreened
unsecured
unsound
unsupervised

unstable | American Dictionary

unstable
adjective
us/ʌnˈsteɪ·bəl/
not firm and therefore not strong, safe, or likely to last:
It is a poor and politically unstable country.
An unstable person suffers from sudden and extreme changes of mental state:
He’s emotionally unstable – you never know how he’ll react.