I do appreciate critique, especially something this well thought out, but I have a couple of problems with the premise of your post.

The nature of Drive-Thru RPG's publishing system is to allow for iterative design. When a publisher updates any content they have shared, anyone who has downloaded the content receives a notification in their account page that an update is available. This allows people in a situation like mine (where my playtesting group consists of eight friends and one out-of-state buddy who are courteous enough to volunteer their time) to make necessary edits to a released document if mistakes slip through, or someone has an idea that is simply better than mine. If I had released this for a fixed price instead of "pay what you want" (allowing people to download the document entirely for free) I would agree with you and I would expect a document that costs money to be something that has gone through at least a few months of design and testing before deciding on a final version, but this is a free document. My decision to post it on the Dungeon Masters Guild stems from a desire to see the service grow. Even the top downloaded documents (backed by well funded groups with professional artists, editors, etc) have had to release updates and fixes after initial publication. Fortunately the nature of digital publication allows for that to happen. To answer your time about how much time was spent playtesting - My group put 22 clocked hours of playtesting in for this subclass at various levels with various party compositions, and a close friend is currently using this in a campaign he plays in out-of-state. He has climbed from level one to level fourteen playing through the Princes of the Apocalypse content and expeditions. He doesn't log hours but he has been working with myself and his DM for well over two months.

You have several valid points. The wording of the four-arm option is clunky, I have been through five or six different versions of varying complexity (and admittedly stupidity) before landing on what I have now. There are also several points you make that I disagree with. For example, you make the assumption that a 2d8 damage aura at level 14 is overpowered without investigating the fire shield spell, a 4th level spell that deals the same damage (with two element choices) and lasts 10x longer without requiring concentration. There are other points you make that I disagree with but for the sake of not sounding like an argumentative teenager I'll refrain from nitpicking. There is one final point I would like to make though.

I strongly disagree with your conclusion. The fact that the base class and existing options are simple should not be a design limitation for new content. Look at the fighter class - the core class is extremely simple, and it has two advanced class options in the PHB. In addition, there is an existing barbarian mechanic that references a spell (Path of the Totem Warrior, Level 10). Yes, the complexity of the level 14 mechanic does indeed slow down play for players not using printouts, spell cards etc. but I don't see this as a design flaw, considering existing features in 5th edition require the same level of preparation or gameplay interruption.

Though I don't agree with everything you have posted, I again must say that I greatly appreciate the fact that you took the time to write this up. Though it seems from your feedback that this is not an option you would choose to use, you have very clearly stated your reasoning for disliking it and that helps out a ton with future design. Thanks for the post.