I have a Lenovo P620. It's a bit noisy, but not intolerable. I haven't had any hardware problems. Some people have reported problems after installing a Lenovo firmware update, so maybe don't do that (I haven't).
It may not be compatible with all graphics cards that you would think it would be - I found that a GTX 780 didn't work for no identifiable reason. Possibly, they've disallowed all but "professional" GPUs. (An NVIDIA K40c did work, and it has the same "Kepler" architecture as the GTX 780.) I currently have one AMD WX2100 GPU (for actual display use) and three NVIDIA A4000 GPUs (for compute). I had had four A4000 GPUs, for which there are four suitable 16x PCIe 4 slots, but found that they wouldn't all run at full speed due to cooling issues (this is a bit of a problem even with just three). This may say more about the A4000 than the P620, however. The 1000 Watt power supply of the P620 is useful if you want lots of GPU power.
The AMD Threadripper Pro processor is powerful, but single-thread performance is a little bit below the top Intel offerings. It has eight memory channels, so if you populate all eight DIMM slots with memory cards, you can get a lot of memory bandwidth. Note, however, that the NUMA memory hierarchy makes fine-grained communication amongst processor cores slower than on some Intel machines.
I bought the P620 to use for ML and other intensive computational tasks. It may be the most powerful affordable option for that. For general desktop use with a bit of gaming, there are probably better choices.
As is generally the case for all compute vendors, it will be a lot cheaper to buy a minimal system and then install memory, SSDs, HDDs, and GPUs from a third-party source, though that of course is more of a hassle and could produce compatibility problems.