In situations where we've pushed early game and are dominating, I'd like to push down the nexus turret. However, after taking the inhibitors it's fairly easy for enemies to lead minions to the fountain turret for instant death. Because of this, it causes you to tank the nexus turret if you want to take it. And while you're attempting to break it, many enemies can harass and attack you while within the safety of their fountain or take two steps back to heal HP/MP enough to head back to the fray.
There are several ways enemies could display you into the fountain for a near instant death as well. Urgot, in particular, can surpress you into the fountain turret while attacking the nexus turret.