If someone is offering a high interest, very liquid investment with returns so high that you are better off taking a larger mortgage, and putting the extra money into the investment, there has to be a hidden risk component: Otherwise, why would anyone issue mortgages, instead of investing in this very liquid instrument? It's too good to be true.
It's not unlike the people that spent money ordering cryptominers from butterfly labs: The expected return from buying them (pay the whole thing off in two months, and afterwards it's all profit!") just doesn't line up, regardless of the justification. Extraordinary returns exist, but they either require that you discover them yourself, or have to carry significant risks. So when someone is selling the opportunity to you, and you are told there's no risk, you have to know you are being lied to, no due diligence necessary.