Well, Mayer faced tremendous reputation risk if she failed. Now that she has failed, she is not held in remotely the same high regard that she was prior to taking the helm of Yahoo.
She's also been implicated in some of Yahoo's more embarrassing missteps during her tenure.
So it's important to weight the parachute against her annual salary in a "safe" job for 3-5 years. It may still seem overvalued (or undervalued) but the price was reached through negotiation.
It depends on your market power when negotiating your contract. Most employees don't have enough market power to negotiate anything more than a week severance pay.
She's also been implicated in some of Yahoo's more embarrassing missteps during her tenure.
So it's important to weight the parachute against her annual salary in a "safe" job for 3-5 years. It may still seem overvalued (or undervalued) but the price was reached through negotiation.