One thing that nobody here appears to have mentioned so far.....
Don't go burning your bridges and telling your soon-to-be-ex-employer how much they stink if there's going to be the slightest chance that your future employer(s) or other people with a need to investigate your past might approach them for a reference.
Tell your employer how much they stink, and you might find they either refuse to give a reference when asked, or otherwise they might just write a nondescript one phrase boilerplate that just says "we can confirm X was employed by us between dates X and Y".
Believe me, I've been there, in the employer's seat... those employees who left on bad terms were always the ones I just happened to be "too busy" or "unavailable" to provide references for !
