Blackmagic has their Video Assist 7" 12G recorder which can record 4K 10-bit 4:2:2 to SDXC cards or externally to a USB-C SSD, but its base price is more than the Ninja V and isn't really optimized for working with Canon the way the Ninja V is.
For all that it does the Ninja V is pretty much a steal. There are a lot more expensive external monitors that don't even record (looking at you SmallHD) out there. It's certainly an expense, and the expense doesn't stop when you buy the monitor, you need cables, batteries, and SSDs and such. but well worth it if you need to record longer than 30 minutes continuously, want to shoot 10-bit (on an EOS R) or just want to benefit from the ease of editing ProRes or DNxHR. To me the biggest downside is you'll need a LOT of storage space, the output files are HUGE, though you can use ProRes LT for smaller files with pretty much no loss of quality.