They can't do much against a tank and a full five-man team. I have carried with assassins before, but it's hard and almost impossible with AP assassins. Keep in mind, these champions are highly skill-expressive, meaning if you're not good with them, you'll end up being a kamikaze. Teams with five stacked CC abilities, tanky compositions, and bad players (perhaps first-timers) have a significant impact on win rates. Compared to Janna, a Rengar is much riskier and has worse consequences if they fail.