We've purchased two cars using Plastiq over the years and haven't had a problem. We use them for our mortgage each month, too. For the last year or so you've been able to get 3x points using your Chase card with Plastiq on certain transactions, so that article makes more sense when you consider that.
OmegaZ - Did you send your checks to the right address that Tesla gave you? When we bought our Model X using Plastiq, we learned that we had to send our checks (we split it up between different cards to maximize rewards) to different addresses given to us by Tesla. It was really easy to change them on our Plastiq account for each one. Just curious if you were sending it to the right address since you get a confirmation when they've been cashed by Tesla.