Agreed. Our business is looking to implement Shopify POS for our front-end. I've had to build some supplemental SQL DB tables to manage partial-payment transactions. In our world these would be layaways of on-hand items and special orders for getting items we don't stock. Using the API, I can port out the details and get at tight reporting, tracking, and analysis.
So far so good, although being able to track the status of these "special orders" would be more effective if I was able to interact with the comments timeline from the Rest API's Orders endpoint.
The only saving grace is that I can at least query this via the GraphQL API. See https://community.shopify.com/c/Shopify-APIs-SDKs/Using-the-order-timeline-on-the-admin-API/td-p/439... for details. Can't write to them, but this is better than nothing!
Did you see my post just prior to yours? This data is exposed in the GraphQL API. You cannot add comments via an API, but you can pull the existing ones that were created in the GUI. If this doesn't suit your requirements then I hear you. Enhancements like these are really just building out what should be existing functionality in a standard cloud-based POS.