This is because "native" support of these configurations often put additional vertical PCI-E slots physically in front of the horizontal PCI-E brackets
These configurations look fantastic, however, what manufacturers neglect to tell the average consumer is that in these configurations, GPU's with air coolers (98% of the market) are too close to the side panel and as a result, suffer significantly from increased temperatures which results in loss of performance. Gamers nexus, a Youtube Tech channel has time and again demonstrated that these vertical GPU mounts significantly affect the performance of the card. There are two acceptable solutions to this problem however.
The first would be to used a watercooled card. In many cases, watercooling a GPU in general is not the best solution, because (unless you have a very high end card) it will not see any significant benefits to performance on mid range cards, is relatively expensive, and often "doesnt look as good as the air cooler".
Option 2 would be to implement what MNPCTech is selling (or what the Lian Li PC-011 Dynamic comes with) which is a Vertical PCI-E adapter that replaces the horizontal brackets on your motherboard. This has the benefit of moving your GPU away from the side panel, allowing proper airflow while still maintaining the desired look. The downside is that other than the slot the GPU riser cable is in, you cannot use any of the other PCI slots on the motherboard, as the GPU is now blocking both the PCI-E brackets as well as the slots themselves.
Another downside is that some cases actually have extra reinforcing bars along the PCI-E brakets in the back, meaning you'll have to (in some cases permanently) Modify your case to support this configuration!
To conclude: Anyone considering using a Vertical GPU bracket should carefully look at all the pro's and cons with their configuration and determine if Vertical GPU's are right for them.