Yes, this is correct.
Third party services introduce a counter-party risk. I have been saying this from day one. I no longer recommend block.io, even though I never had problems with it myself, because I have seen some complaints about the service.
My first plan was to create a plugin that can directly interact with full node wallets. Third-party services were an after-thought, once I realized that most web “developers” don’t know Linux 🙂
I am aware of Blockonomics. Unfortunately, developing a new coin adapter is out of the question for me at this time.
My current recommendation is that people need to first understand the trade-offs, and decide if they want security or convenience. See HOWTO: Choose between available coin adapters.
with regards