Microsoft Flow Planner connector currently implements a subset of Planner REST API. But not all: Get more task details, create plans, add/modify categories, delete tasks, add checklist items move a task from one bucket to another,... are available in Planner REST API but not in Flow Planner Connector yet. So, in such circumstances, we have no option but to use Planner REST API Calls: either by using an HTTP action in Microsoft Flow or by implementing a Custom Connector. But none of these options are straight forward. We need to implement some sort of Authentication and Authorization techniques, or obtain some sort of Authentication and Authorization related data so that the Flow can access Planner seamlessly.
A similar problem arised with Sharepoint, and since May 2018 Flow offers the option to invoke "Send an HTTP Request via Sharepoint" action block, so you just need to know about JSON and Sharepoint REST API, without having to deal with Tenant IDs, Client IDs , Client Secrets, and other Authentication/Authorization stuff.
We are requesting a similar approach for Planner, i.e. a "Send an HTTP Request via Planner" action block.
If this idea is finally implemented, a bunch of Planner related Ideas representing more than 850 votes that are currently under "New" status can be marked as "Completed":