YC companies seem to be extreme in some form or another, good, bad, and ugly. Do write it up on Glassdoor. It's a record and something people will check, investors, applicants, and competent HR.
YC itself probably can't even act on it if they wanted. They invested money and if the company was bad enough, they'll die on their own. YC tops up money in a non-signal manner (only when others do). So even if they wanted to back out of an investment, they likely wouldn't.
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.
A good YC company would be glad you provided feedback they can use to make the process better. Other startups YC or not can learn.
Stories like yours are common and we all can relate. Honestly it doesn't sound all that bad, the CTO is letting you choose the stack and run with it. Distracted CTOs are much better than owners who think they are programmers (who get drunk and commit changes at 2am that break production). Hiring and firing/quiting is common in a startup. You need to build a team that fits together and getting that right is hard.. things usually stabilize.
Is this your first startup?
If you want to let YC know, do it in private.
If you want to help other people avoid this startup, weigh the satisfaction of doing that vs the fact that people are generally wary of hiring whistleblowers.
If you want to air the laundry and remain anonymous, use something like Glassdoor. But your opinion won't have much weight unless it's backed by others.
Without being shamed or accused of hurting someone’s abstract feelings.
For instance, this post would make much more sense and impact if OP would just write
“hey guys, cto of YC funded company ABC, in my opinion, is doing awful job and i as an employee have terrible experience. Here is why …. What do you suggest?”
While there may be a moral argument around protecting potential new employees and warning them not to join. Or to encourage YC to better monitor their companies....
Having said that, to have an impact you would really have to go out of your way to make a point...there is a cost associated with that especially if you dont get the outcome you would like. Some people could potentially view you negativity or that you rat people out. Personally, I would not do anything unless it was something big like fraud or sexual abuse/rape because there is a moral argument and then the board/investors would really pay attention. If it is just bad culture, the board/investors could just turn around and say they are aware of it and are working with the leadership on this.
Like anyone else, YC will do whatever is in it’s own best interest, not yours. So be careful and give much more thought to any action you take.
I often have grandiose ideas that I will give some glorious speech full of witty put downs, but then I get out of the shower and realize it wouldn't play out that way in reality!
If it is truly untenable just get a new job and it sounds like you at least got paid and have some good stories to tell
Startups are very much like President Trump's administration. Everyone is all in on the golden goose and they'll slowly twist themselves in circles justifying and doing unseemly things, to hold on to the power, and especially the payout, at the end of the tunnel.