I was reviewing the Trail Head - Process Automation - Figure Out Which Tool to Use, and stumbled upon the statement that says:

"Process Builder includes almost all the functionality that’s available with Workflow, and then some. In fact, a single process can do what it would normally take multiple workflow rules to do, which helps you avoid limits.

There are only two things that you can do with workflow that you can’t do with [Process Builder] processes: Configure actions to be executed at different times."

Please correct my understanding but hasn't this limitation effectively superseded by Winter '16 release? It says below:

"Schedule Multiple Groups of Actions

You’re no longer limited to just one schedule when you configure time-dependent actions in the Process Builder. With multiple schedules it’s easier to optimize sales responsiveness, automate follow-up on important cases and opportunities, and support your business needs all within a single process."

I'm following work.com workbook but having it prinstalled from Salesforce 1 for admin guide so some parts need some modifications still. I have encountered this tricky issue.
Schema is such:
Warehouse (Master) - (detail) Merchandies (Master) - (detail) Line Item (detail) - (Master) Invoice

No roles in place yet

OWD:
Invoice  Private  
Line Item Controlled by Parent
Merchandise Controlled by Parent
Warehouse Public Read Only

Profile:
Merchandise Read
Warehouse Read

Permission set:
Invoices Read, Create, Edit, Delete
Line Items Read, Create, Edit, Delete
Merchandise Read
Warehouses Read

I can create Invoice record. Then on Line Item related list I click on New. Input webform opens. Invoice field is prefilled as open from related list and then I browse any Merchandise (owned by other user) and click save and I got this error:

"Insufficient Privileges
You do not have the level of access necessary to perform the operation you requested. Please contact the owner of the record or your administrator if access is necessary."

Why is that? When I change OWD for Merchandise to Public Read/Write it lets me save Line Item record but why? Based on my definition order of relationship is such that primary is between Invoice and Line Item.

I'm lost a bit here ir it's so simple that I just don't see it.

Please help.