I believe personal/guild island crafting is only "worth it" if you're crafting a lot of T4 or T5.
If crafting T6+, then you should probably use stations in town... unless you just really hate the cartel, and would rather spend more money than pay them at all.
That's about it. That is why my crafting stations on my island only go up to T5. (That and the exorbitant cost of T6 and above stone) Everything of T6 and above and a few T5 goes to town. The only slight exception is in the refining specialty for your linked city. You get almost as good a return on your island for that so it helps a bit.