So, security-wise, the bot should be pretty safe.
Only users can authenticate themselves to the bot using a password, so if you don’t have one, you shouldn’t stand a chance.
The data is sent to Telegram using a WebHook over HTTPs, which also makes it happen encrypted. Telegram itself does not have end-to-end encryption, but it should still be encrypted quite well. You can read more about this online, Telegram uses their own way to encrypt chats.
The data may be stored on Telegram servers, whether you trust Telegram in this regard is up to you. I don’t see any problems in our case.
The code of my bot as well as of the libs used is open source, and you are welcome to read it to check if there are any concerns for you.
Of course, any feature added to a system can somehow open up an opportunity for attackers to do unanticipated things. As of now, however, I am not aware of any security flaws. If anyone finds any, please let me know and I will try to close them.
You can also write me a private message and tell me how and what you would improve the bot, I am open to updates if they seem useful to me.
I would also like to know how the installation went for you and if you had any problems. 