... I am sure there are security experts who have vetted Keystone and its products, and you will need to trust that they did a good job. It probably hasn't been put under the miscroscope as close as Trezor, for example...
Despite Trezor being trustworthy, I don't want to rely solely on it. I have purchased a Keystone 3 Pro, and will store ETH along with my existing Trezor Safe 3. This will help diversify risks. Keystone uses QR codes, which feels more secure than USB.