from
reddit by luke-jr taken from that article:
1. It's a website. Even if you download it locally, you're setting yourself a habit of putting private data in your web browser.
2. It's Javascript, which is an extremely poor record for security and crypto, and is super-flexible to the extent that it can be hijacked in subtle ways (think browser extensions that quietly redefine how basic mathematics works).
3. It encourages either address reuse (which has no shortage of problems, as /u/sQtWLgK pointed out), or managing multiple keypairs by hand (which is liable to accidental loss, since key management is excessively complicated and humans screw up eventually).
although i have to disagree about calling the "tool" unsafe just because users may use it wrong (points 1 and 3 and partly 2). for example if someone is using the website then they don't understand what this tool is for, and for these types of users no wallet or other tool is safe because they can lose their money just the same way.
or regarding #3 paper wallets have a clear purpose, they are meant to be used as a cold storage which means when you have a certain amount of bitcoin and want to "store"
that for a long time. again if you are reusing that address then
you are using the tool wrong.
as for point #2, if the source code is run on a clean and offline computer (like a live Linux from a DVD) then i don't see how this could even be an issue.