Yes, welding cable uses has fewer strands than the high-quality audio power cable. But the welding cable is still very fine stranded, bends easily, and Ive yet to come across an install that needed more flexibility.
Remember, when we talk about 'welding cable' we are referring to the cable running to the stinger (the main cable to the actually welding tip you hold), which gets dragged around alot and man handled, so they make it very flexible. Its only considered coarse strand here because its being compared to audio cable which is almost ridiculously fine stranded.
Show me an install that required the power cable to be tied into a pretzel with 1" radius curves and I'll show you an install that requires Kicker's high dollar cable. Otherwise, welding cable will work just as well, for a fraction of the cost.
Im not greatly concerned about the jacket. I always run any substantial sized cable in conduit of some sort anyway.