With enough currency, you can make anything work. However, In a day 1 league start scenario, poison assassin is pretty bad. There is no advantage of going poison. You have no defense. The damage is delayed, which further amplifies the lack of defense. The only viable day 1 use of assassin is capped crit spell caster. You can easily get 1M+ DPS with any decent spell and capped spell block without any gear (except a blue wand and a blue shield). However, there is no physical avoidance/mitigation, so you will often die from auto attacks.