TLDR: Should I report my boss' shortcomings to HR after quitting (but before actually leaving the company)?
I have been working as a senior developer at a large IT company for the past 4 years. Somewhere in the middle I wanted to change my team because I felt stuck in the old one. Around the same time a reorganisation was going on, so also I got a new boss. I never heard what happened to my former boss, only that he doesn't work in the company anymore. His dismissal was never communicated.
I approached my new boss because of my plans to change the team, which he got in motion very quickly. I also asked for a salary raise because my current salary was clearly below what others in a similar positions earn (he even confirmed this). He snapped at me, saying this is the wrong time to ask for a raise, that if I don't like it I should look for another job, that he was here to stay and that we wouldn't get rid of me so quickly. I was surprised to say the least, not the reaction I had expected.
I had several other conversations with him about getting a raise/promotion, those were mostly in a calmer tone. He promised me a "substantial raise" the following year, if I proved myself in my new team.
I did prove myself in my new team. I quickly grew into a source of knowledge for the new team, and acted as a mentor for others. Every year we assess ourselves within the team, which can be relevant for a promotion/raise (at the end however, the management decides). My teammates recognized the work I do and unanimously gave me only the highest ratings. My boss knows this and even confirmed he only heard good things about me. However, still no raise, just another comment about probably being better off at a new company.
I have mixed feelings towards my new boss. On the one hand he can be quite sociable and helped me effectively with my internal change. He also always took the time for a talk when I asked for it. On the other hand, I think he never really cared if I stay or go. Nothing really changed salary-wise. He also gave me hints several times that it would be best for me if I quit, which I wouldn't expect from my boss. He also stalled me with false promises about my promotion/raise. I should also mention here that my boss has 200+ employees to supervise, so I have to be lucky if he even knows my name. So I know it's nothing personal, especially since I head others complaining about the same issues.
I meanwhile drew the obvious conclusions and put in my notice. We have an internal tool for this, allowing to give a reason for quitting. I selected "lack of trust in supervisor". I was contacted by HR shortly afterwards, scheduling a meeting so we could talk about this.
My question is now: Is it a good idea to spill my guts to HR or will this backfire at me in any way? (e.g. as a bad testimonial)