Eye friendly?
The biggest influence over eye friendliness of any screen is (a) properly corrected eyesight and (b) screen resolution. You have control over ensuring that your eyesight is properly corrected (using prescription glasses or contact lenses as appropriate). As for iPad screen resolution, in particular iPad models with "retina" displays, individual pixels are too small to individually differentiate; images are extremely sharp.
OLED screens fundamentally offer much better contrast and brightness control than LCD screens.
iPad LCD screens are reliant upon their backlight, the technology used to achieve this being different with some models...
miniLED backlights use tiny LEDs that are distributed directly behind the LCD panel - the LEDs being grouped into thousands of individual dimming zones, the brightness of individual dimming zones being independently controlled. As such, screen brightness contrast is significantly improved over traditional LCD backlighting systems.
iPads without miniLED backlights are edge-lit by dozens of LEDs arranged around the periphery of the screen. The edge-backlight is distributed across the rear surface of the LCD screen by a backlight diffuser layer - this providing even backlighting for the entire screen - brightness being the same for the entire screen; screen contrast is not as good as miniLED or OLED screens, as the LCD alone cannot produce true black.
In summary...
Good - Apple Liquid Retina (LCD display)
Better - Apple Liquid Retina XDR (LCD display with miniLED backlight)(e.g., 2024 iPad Air M2 and fifth/sixth generation iPad Pro 12.9")
Best - Apple Ultra Retina XDR (Tandem OLED)(e.g., 2024 iPad Pro M4)
