What types of data are safe to put on the blockchain?
In most of the implementations done on Kaleido's infrastructure, data in its raw form is stored off-chain and referenced on-chain via a hash of the data. It is Kaleido's recommendation not to put any confidential or sensitive data on-chain due to the fact that the blockchain represents an immutable and permanent ledger of information. Depending on the use case, leveraging IPFS, Document Exchange and/or FireFly Data Exchange as delivery mechanisms for off-chain messaging in parallel to pinning an on-chain hash of these files is typically the preferred path for referencing data on-chain.
It is a best practice to take a minimalist approach with the data that you are storing on-chain. There are flexible models such as the proxy-enabled upgradeable smart contracts that let you increase on-chain visibility over time without losing any existing state.