I don't have a horse in this race, and I have no recent experience with them, so just my 2¢?
Sounds like their engineering is on track (useful features, no fantasy).

It's their financial plan that makes me nervous. A business that size should have no problem floating their R&D costs from their bank. The loan interest is part of the overhead - everyone does it (the garment district uses nothing else for 100 years).
When they ask for that much money, they're telling you they can't pay their light bill. When it's still not done, 2 years from now, they'll ask you for another $1000 toward the new 2019 price of $5500. You already agreed to "no refund".
Trust me: the money isn't in escrow, and they will refuse any request for a financial statement ("where did the money go?"). It went to rent and salaries, like it always does.