The following are some noteworthy points to remember:
Don't perform the DML operation inside the Loop element. It will easily hit the governor limit, that is, System.LimitException: Too many SOQL queries: 101.
You can't refer a Flow as Subflow if both types are not the same, either Flow or autolaunched.
You can only use the Wait element in autolaunched Flow.
A Flow runs in user mode and Process Builder runs in the system mode. Let's take an example if you are trying to update Opportunity the next step field:
If you use Process Builder to achieve it. If the running user doesn't have access to the next step field, Process Builder will be able to update it.
If you use Flow to achieve the same thing and the running user doesn't have access to the next step field, he will get an error.
If the Flow doesn't have a Start element, you won't get a link to active the Flow.
DML operation on a setup object is not permitted after you have updated a non-setup object (or vice versa). If...