Blower style coolers can be noisy and the card can tun hotter. But they push the air to the outside of the case.
You have to check that a card works with a Mac (it seems XFX usually does not because of an incompatible VBIOS, for example).
There are some eGPU enclosure deals around $200. They are much better than what you could get for that money some years ago, but maybe you would not be able to fit a monster).
Okay, yes, I gotcha on the blower style coolers. I was looking at an MSI RX580 that NewEgg had on sale, but the common complaint was that the card ran hot due to the single blower style fan.
It's a shame that AMD chose that configuration for the FE version of their Vega 64 as that is the only card I know of that contains 16GB of HBM. I suppose 8GB is enough, but for the cost that Sonnet Technology is charging, $1200, for the card and the enclosure, I find it very reasonable given that that card was somewhere north of $1500-$2000 back about 6 months ago.
I suppose that this card might now work with the Mac, however, it is essentially the same GPU that Apple is putting in the iMac Pro, so compatibility should be Golden™.
I restrict my longings to Sapphire and MSI as those are being advertised and sold by Sonnet Technology, although I believe Asus and Gigabyte should be good. I have heard many bad things about XFX and although Apple recommends the PowerColor DevilBox, I would never buy one due to PowerColor's rather dubious record here in the USA.
The Sonnet eGFX Breakaway Box 350 is $199, but it can only support a subset of cards due to power requirements and as a result, it cannot provide 85w of charging for my 2016 15" MacBook Pro. The 550 is a better way to go, but the 650w is the one that I want, although even it is not guaranteed to support the 7nm Vega Instinct consumer GPUs slated for 2019 if the rumors about power consumption are to be believed.
Herein lies the Achilles Heel of the eGPU market and why the BlackMagic boxes may fair better as they work right out of the box, while the roll your own approach is still sort of Wild West right now.
PS - Prices for the RX570 and RX580 may go down a bit once AMD formally announces the Radeon RX590.