The Adreno 530 GPU integrated into the Snapdragon 821 is one of the most powerful graphics processor one the market right now and it was design to do some serious graphics computing work and to boost the 3D performance of the device, which is especially useful for games of course. There are already many smartphones based on the Snapdragon 820/821 Soc that come with this integrated GPU, including among other: Samsung Galaxy S7, Galaxy Note 7, Xiaomi Mi 5S, Google Pixel XL, OnePlus 3T and many others. The performance varies between devices. For example, in the Mahattan 3.1 benchmark, the Sony Xperia Z5 Compat with its Adreno 430 processor performed better than the OnePlus 3T and Google Pixel with its Adreno 530, but tight to the 821 Snapdragon, the 3D performance is going to be superb.
The most powerful GPU would definitely be the Adreno 540 in the Snapdragon 835 which is said to makes its debut in the Samsung Galaxy S8. I can understand why some people might be disappointed that the ZenFone AR doesn't come with a newer SoC, it's 2016 after all, and for such an advanced high-end device with Tango and Daydream support, it would have been amazing had it came with the Snapdragon 835.
It's not just for performance reasons, but the new processor should also decrease power consumption, thus reducing heat and make the battery last longer. According to early benchmark posted on various website, the Adreno 540 GPU has shown around 30% better performance.
I don't see it as a disadvantage, and in fact, it's a very powerful GPU that can run any game you throw at it and make all apps runs smooth and fast. It will all come to what price the Asus ZenFone AR sells for. If the price is right, no one will complain, but if comes at a very high price, there will some people that might convince to buy one of the more anticipated high-end smartphones like the Galaxy S8 and LG G6 instead of the ZenFone AR, regardless of its Tango support, which is still in its early stages and doesn't have a lot of content anyway.