Coinbureau did a thorough analysis of the wallet. It was not in their video on top 5 of coldstorage wallets because they had not reviewed this prior to releasing the video. But they are very positive about it. Particularly the way the seed is generated, is unique and guarantees that there is no backdoor. For the other cold storage wallets, I am not convinced of this. If NGRAVE is bad, I wonder why it has the highest security certificate.
Note that Coinbureau is making paid crypto reviews and articles, posting ref links on his website, etc.
He also earns percentage of every Ngrave wallet sold from his link, and that is much more than he would get from some cheap wallet.
I would never recommend anyone to buy any closed source hardware wallet, and there is nothing super-special about Ngrave seed generation,
and I researched seed generation in most hardware wallets here:
https://bitcointalksearch.org/topic/seed-generation-in-hardware-wallets-5317199Let's assume that their so-called secure OS was indeed very secure, then why they're still afraid to make it open-source
Same reason why ledger is not doing it... I am sure many people would discover a bunch of flaws and exploits, showing how ''secure'' this devices really are (not).