I definitely agree with Ericka, and actually I always felt the question as a matter of starting a conversation.
I would not define the spas as a downside of Amans. The level of service and locations may be different but in my experience it has been always outstanding. On the other hand I experienced in Banyan Tree's spa in Ringha a lack of hygiene (e.g. used slippers which I had to ask to be changed, sheets which didn't seem to be spotless) which is absolutely unacceptable (I could not relax during the whole treatment...).
The shops are not always matching with the same level of quality you would expect from an Aman (I found Amankila's boutique quite uninspired, while in Amanpuri they had amazingly cool jewels and artifacts).
The only significant flaw I have experienced with Amans happened this summer in Amankila where I have found in my room all the letters and welcome correspondence addressed to myself and to my former girlfriend (!). Luckily, the guy who took me to the room understood the point in a second and managed to make the papers disappear before my fiancée noticed anything...