its just you get around 25-50% less performance compared to a laptop graphics card. and a glaptop graphics card is already behind 10% of a desktop graphics card.
to put it percentages
desktop gtx 1080 = 100%
laptop gtx 1080 = 90%
external desktop 1080= 50%
this is because tunderbolt 3 is 3 times slower than pci-e x16, thats just theoretical.
then there are the software problems, just alot of games dont like it.
pricetag 800 dollars?
but yes it works.