WordNet sense 1 (not firm or firmly fixed; likely to fail or give way):

insecure

WordNet sense 2 (not secure; beset with difficulties):

shaky

WordNet sense 3 (fraught with danger):

parlous, perilous, touch-and-go

WordNet sense 4 (involving or causing danger or risk; liable to hurt or harm):

dangerous

WordNet sense 5 (affording no ease or reassurance):

unstable