Just keep in mind that you may end up having your own identity revealed - especially if your commentary appears to be too caustic, or to focus too much on petty mistakes instead of the bigger picture (everyone screws up, or says something stupid now and then - it's just a question of degree). And also, please take care to avoid speculating about matters you can't directly verify (did the CTO literally "not look at the stack for 5 months", and if so, how can you tell?)
Also, you may want to make your report (at least plausibly) anonymous -- that is, don't name the company, or give details about the line of business. That will spare you from charges that you have an axe to grind, and people will be less likely to take a sympathetic position towards they company (as they sometimes do when companies are ratted on).
But if you go that route - do mention that it's a YC company (because it's definitely in the interest of the readers here to know).
So a good strategy may be: take some time out to compose (without making it too long - otherwise it will appear as a "rant"). And if you can, show it to a friend who can gut check it for you. Then if the two of you feel good about the content - go ahead and post.