Ports are way too wobbly unless you use your laptop as desktop. I love ability of using 2 external displays with MacBook keyboard and trackpad just right there in the center. But move the computer a centimeter and displays disconnect. Moderately painful!
This. I regularly drive my 4K monitor at 60hz from my personal rMBP (2013 with dGPU) and work rMBP (2015 without dGPU). Interestingly the first MBP to DP cable I used didn't work, but the second did.
An external Thunderbolt 2 GPU will only run at the equivalent of PCI x4, while Thunderbolt 3 eGPUs will do enough bandwidth to have full GPU performance. eGPU driver support was added to macOS Sierra, so hopefully the new Macbook Pros with USB-C / Thunderbolt 3 will support eGPUs nicely.
The technology is really not there yet. I have been considering changing my setup to a 13.3 light notebook with long battery life + an external GPU, but I'm still waiting on the technology to become mature.
There are no mass-market thunderbolt GPUs with fully-functional mac drivers at this point, are there? I've been looking and at best there are unsupported hacks.