I've fallen for a trap like this before so I feel adequately prepared to answer this.
Let's call it,
The "Brilliant" Engineer That Needs Your Funds Trap.
There are two possibilities... and the shitty part is that if the engineer is smart enough, you'll never get any sort of conclusion. You'll never really know which possibility is the real one.. You'll always be in a state of wonder.
Possibility #1) They are genuinely smart and came up with a solution that seems correct on paper. They might have made an error somewhere, but they're too caught up in their visions of grandeur that they
never double-check their work. They'll also never give you enough information to check their work because their ego won't allow them to be proven wrong. They'll always give you an excuse for why they can't provide more info.
Possibility #2) They are a genuinely smart but evil person that wants
you to pay their bills. They will promise you the world... but it's always
just out of reach. They need more funding. They need to land a contract with an impossible company to work with. They need six more months. They need... [
fill in the blank]
What should you do?
Walk away and don't look back. Ever.
They are energy and money vampires.
If they were legitimate, they wouldn't be talking to you in a place like this. Instead, they'd be like
Lonnie Johnson who patented his invention and then
went straight to the people who needed it.
You've been warned!