1 Had only one color
The graphics seen through the Nintendo Virtual Boy were in one color only, which was red. Many people expected to see richer colors.
2 Low quality graphics
The quality of graphics of the Virtual Boy wasn't good. Many customers found it disappointing compared to the hype that was made about the device.
3 The device wasn't portable
The Virtual Boy wasn't a portable device; it was fixed on legs and couldn't be moved. The player had to sit on a chair while playing.
4 No motion detection
The Virtual Boy didn't have a head motion detection. This made the experience seem less like VR and more like playing a game in 3D.
5 The height wasn't adjustable
The height of the Virtual Boy wasn't really adjustable, which made it hard for people to create the proper setup that allows them to play comfortably. (See Why Rise of the Tomb Raider was initially considered a failure?)
6 It caused sickness
The Virtual boy resulted in sickness and nausea. Some players who ignored the sickness played until they vomited.
7 It wasn't really VR
The Virtual Boy wasn't really a virtual reality device, but instead, a viewer that could show 3D graphics that weren't of a high quality.
8 Health risks
The text on the Virtual Boy warned of health problems that could happen if a person played for too long. Problems such as headaches, eye problems and seizures were mentioned.
9 No great games
The Virtual boy didn't have games that were impressive enough to motivate people to buy the device. Many of the games would have been better suited for a normal 2D console. (See Why Super Mario Run is bad?)
10 The setup was uncomfortable
It was hard for people to operate the device while sitting comfortably due to ergonomic limitations. Nintendo promised improvements but the device was discontinued before that happened.