I actually addressed that in saying that Aug met a "limit" as to what he could make. Not to mention, for Shiroe to use the Scribe Succlass to use Global Class Magic, it took Phantasmal-level items. That basically sets the precedence of the most powerful creations require the most powerful reagents. With that said, I believe it's entirely feasible to limit things like batteries, durability, damage, MP consumption, etc. via the reagents used to create and upgrade creations. In plenty of MMORPG's, you begin creating interesting items at the mid-level, but the most powerful ones are an entire world more capable. So, while I can understand your point in crafting subclasses, I also took measures to counter that. A large plot point of the character will be not only attaining superior materials, but also getting to work with Blacksmiths and other crafters to assist him in his ventures.
Using World of Warcraft, for example, the first things you get to make are the tools to make other things; by the first 25% of the crafting tree, you've basically made every tool you'd need, and later you make better versions of them to work with superior materials via actually getting those materials, such as True Silver, Thorium, and even now Draenic Ores. His "ultimate" creation is just a giant, mechanical scorpion, but even then, it has plenty of room to be improved. We also don't know much about the Mechanic Subclass pre-Apocalypse; we know that they were mostly confined to simple machines. Obviously, there's a whole realm of creativity to work with when you're no longer limited in what you can make. All in all, I would imagine he should be able to make just about anything he wants out of iron and steel, but later when he gets access to upgrades, he should get far better materials to work with, increasing magical properties as well. Just imagine the possibilities alongside a Sigilmaker or other roleplaying subclasses. The idea I was toying with were magical prosthetic arms, so if someone were to lose an appendage as is the case in the current canon of Log Horizon, he might be able to replace it effectively and likely even enhance the capabilities of its owner. Another note is that he lacks the financial resources to actually create anything overtly powerful. He would need the assistance of Blacksmiths, Sword Makers, Sigilmakers, Tailors, etc. to actually create immensely powerful creations, so the character would benefit greatly from a trade Guild - which doesn't exist yet.
All in all, I believe it's worth noting that I put a ton of limits on the character and require a lot of resources for him to do just about anything. The fact that a mid-level character uses his subclass as he does shouldn't really be an issue. Most games don't even limit your "professions" or "trade skills" with a level cap; they just limit the resources you can get by the area or mobs you acquire them from. So, with that thought, his subclass skill level wouldn't logically be limited using regular MMO-thinking; his access to materials, gold and other subclass assistance would be.