Security holes in Pix 501??

I have a vendor that I work with that tells me he will not install a Pix 501 on any of his client sites because there are known exploits and holes in the software that make a 501 very simple to compromise.  He tells me he can compromise one within a few minutes and from there can have access to the rest of an internal network.  He won't give me the specifics on this exploit, which leads me to believe he might be full of it, but he says he will NEVER install a Pix 501 anywhere.  He also mentioned that this problem only exists in the 501 series of Pix because he has no problem at all installing a 506e or higher.

So my question to the experts is, are there any documented or known exploits that exist in the Pix 501 OS Software that cause them to be unreliable?  I currently use them in several of my own client sites and have never had any problems but would be very curious to know about this for future reference.