The problem with giant boxes (full of RAM / SSD / Disk) is giant failures and huge recovery times. This is worsened in case of RAM because now every power blip is a full on recovery situation. Have a big enough data set focussed on a single box (or two for backup purposes), your customers are going to blow a gasket the moment one of them go down because workloads usually grow to accommodate available capacity.
Well, you wouldn't run such a server without a hefty uninterruptable power supply system. On your bigger server you can expect a smaller frequency of failures due to fewer points of failure, and can make your system more resilient (rendundant RAM, filesystems, power etc).
FB has a nice paper that talks about this problem. https://research.facebook.com/publications/300734513398948/x...