The black splotchy phenomenon might be due to ineffective mildewcides/fungicides in the stain.
Another possibility. Went to the Penofin website and the hardwood formula page. At the bottom, is the following:
A maintenance coat of Hardwood Finish should be applied within 3-6 months after the initial application or as soon as oil looks depleted. The next maintenance coat should be done again in approximately 10-12 months or whenever oil looks depleted. Over time, the wood fibers become fully protected and the maintenance will become less frequent.
In Penofin's own words, hardwoods should be cleaned and stained three separate times in the first year!
How many consumers or contractors are following that costly and rigorous schedule?
Have seen this type of overall failure more times than I care to count.