The neural engine not having a native programming model makes it effectively a dead end for external model development. It seems like a legacy unit that was designed for cnns with limited receptive fields, and just isn't programmable enough to be useful for the total set of models and their operators available today.
That's sadly true, over in x86 land things don't look much better in my opinion. The corresponding accelerators on modern Intel and AMD CPUs (the "Copilot PCs") are very difficult to program as well. I would love to read a blog post on someone trying though!
I would hope that the Foundation Models (https://developer.apple.com/documentation/foundationmodels) use the neural engine.