One thing that's missing from making custom_json a true killer feature is indexing. If you broadcast (write) a custom_json, that's all well and good. But how do you retrieve (read) it later?

One way would be to buy a subscription to SteemSQL, but that's 40 SBD per month.

Another way would be to stream the blockchain from a certain block height (the genesis block of your app). The problem is, if you ever need to rebuild the data after a few years of uptime, you're going to have to replay all of those blocks again.

Steem Engine NFTs can be used for this instead. You create an NFT, then declare all of the properties your app needs. Then you create instances of the NFT (objects) and assign values to those properties. Then you can query them from the Steem Engine API.

If you goal is just pure data storage/retrieval, it might be a good solution. Plus, if you can figure out how to make the NFT instances useful as tradable assets, that's even better.