Time is the killer
I have transitted DXB 6 times in the past 18 months, each time with a planned 2-3 hour transit.
In reality, due to either departure delays at either end or the inevitable arrival holding at DXB, my transit has eroded to near minimum - usually leaving no time to use the lounge, shop or relax.
If you really want to buy something, better to eliminate the chance of missing out and buy when you are able to ensure that you have the time at either end of the route!
Risk of missing out is quite high in DXB transit.
I have similar experience in AUH.