Does FSX accurately simulate different runway braking conditions?

FSX's ground model is known for being messed up, so I'd say no.

However, PMDG have made an astonishingly accurate representation of the Autobrake system on the NGX.

A definative no, but as mentioned PMDG has done what they can.

yeah a lot of developers have worked around this, FSCaptain, PMDG, the soon to be released Majestic Q400 and I think Ifly