Informant does not support complex recurrence options when using "Recur by Completion Date".

There are some undefined behaviors when you allow complex repeating options on repeat-from-complete.  A lot of those issues centered around what to do if you complete the task "early"...which meant, you had to wait until the exact day that it was due (or wait until it was overdue) to mark it complete.  For instance:  consider that you have a task that you have set to repeat the last Saturday of the month (repeat when complete).  So, for this month, our task would be due on August 27th.  Now suppose you complete that task prior to the 27th (it doesn't matter if it's the day before the task is due or a couple weeks before the task is due).    By definition, you've said you want the task to repeat on the last Saturday of the month using the completion date as the reference point.  So if I complete the task today, then the logic would say, "using August 1 as a reference point, generate a new task that is on the last Saturday of the month"....well, that would be August 27th....the same instance of the task we just completed.  Surely that's not ever the expected behavior, but that's what the rules of the task dictate all the way up until you complete it on Aug 27th, at which point, the next task would be Sep 24th.

Ok, so perhaps the answer to that problem is we say "well, maybe we look at both completion date AND due date and we take the later of the two...because after all, if we completed the task that was due Aug 27th early, then we really intend for 'that occurrence' to be completed, so in that scenario we base the next task off the due date".  Which in many situations works great for custom repeating tasks, but in almost all situations, it's the wrong solution for simple repeating tasks.  Consider this:  you get an oil change every 4 months...this is the classic case for a simple "repeat from complete".  Now, say you are going on a road trip, so you get your oil changed 3 weeks early.  The expected behavior is that my next oil change is schedule 4 months from when I actually complete this task...however if we used our proposed logic of "the later of the two date between complete & due", we'd schedule our next oil change for 4 months & 3 weeks from now....which we almost certainly never expect.