Has anyone had any problem running straps from the top of the lift gate, over the spoiler, and down to underneath the car for either a non-trailer hitch bike carrier or one of those bag type cargo carriers that rides on the back of the car. The spoiler doesn't seem to be strong enough to take much weight.

 

I would be careful with the straps vs. the spoiler. Bikes/cargo gets heavier than you think. Lots of stress on a plastic part.
Consider adding crossbars & using a roof top carrier.
Trailer hitch receiver would be another method. Easy to install & cargo in the box at ground level.