My hunch is that there are worse initial 7% equity holders than YC should control become an issue down the road...which is to say I haven't read about YC screwing people over control, not that founders have nit perceived such actions.
Absolutely but the next round or the one after that might push you over the 50 border and it need not be YC that does the screwing, that could easily be a later investor.
Of course. I guess there might be soft advantages from YC's participation that reduce the probability of getting into a bad situation, e.g. access to investors less likely to throw founders overboard and a network which may make questionable investor behavior higher profile.
Investors would think twice about "screwing" a YC backed company. PG has clearly stated that they would blacklist VCs for various not-so-ethical practices.