WordNet sense 1 (susceptible to attack):

vulnerable

WordNet sense 2 ((of theories etc) incapable of being defended or justified):

untenable

WordNet sense 3 (not reasonable; not showing good judgment):

unreasonable

WordNet sense 4 (incapable of being justified or explained):

insupportable, unjustifiable, unwarranted, unwarrantable

WordNet sense 5 (without excuse or justification):

inexcusable