The following usability issue made it difficult for me to get started with Flowbuilder:
I wanted to add a custom field to Flowbuilder, but the needed ticket field, which we use for internal routing of tickets was simply not available.

The reason is that you can only use ticket fields that are of permission type "Editable for end user":

Unfortunately, this requirement/limitation is not mentioned anywhere and I only found this out after a lot of trial and error - see note in the corresponding form: