I'm disappointed, but not surprised, to find that the support experience for the Defender ATP product has not improved in the last 6 months. I would seriously urge caution to anyone taking a look at this product that expects to operate without a costly "Premier" support contract, which depending on your organization's size may be unattainable no matter how deep your pockets. The "Professional" support is anything but.
I currently find myself waiting for the initial callback after a case was opened (as a transfer from the Intune team, who is quite responsive by comparison) on Thursday last week. It was opened as a severity rating C with an expected response time of 8 hours, but it's Monday and I've not heard a thing. The most infuriating part of it all is the total absence of contact info for me to proactively engage with the support team. My only choice is to sit. And wait.