SD CardsAnything with an SD or Micro-SD interface requires a 512 byte SRAM buffer to communicate with the card.
PixelsEach pixel requires just 3 bytes of SRAM to store the color. But those bytes start to add up when you have many meters of strip or a large array.
On an Uno, you might be able to drive as many as 500 pixels - assuming you don't use much SRAM for anything else.
RGB Matrix DisplaysLike the pixels, these matrix displays require several bytes of SRAM per pixel.
The 32x32 model requires about 1600 bytes of SRAM. The 16x32 needs around 800 bytes.
Monochrome OLED DisplaysThese only require 1 byte for every 8 pixels, but due to their high resolution, there are still a lot of pixels!
The 128x64 version requires 1K of SRAM
The 128x32 uses 512 bytes.
ST7565 LCD DisplaysLike the monochrome OLEDs, they only need 1 byte for every 8 pixels, but they have a lot of pixels, so they require a 1K buffer.
e-Ink DisplaysThese high-resolution monochrome displays support some basic drawing modes that do not require in-processor buffering. But to enjoy the full capabilities of the Adafruit GFX library, a SRAM buffer is necessary.
The 2.0" version of this display requires 3K of SRAM, so GFX is only usable with a Mega.