These past couple of weeks, I've been sewing up a coupla dozen personal peace flags for a long overdue pumpkinbear etsy shop update (I have to take photos outside for them to look at all decent, and in this weather...brrr!), and if I had to use my uber-expensive Jacquard Neopaque fabric paints on every single PEACE, I would be out of business!
That's not to say that you should never use fabric paint on a non-wearable fabric project, or should always use fabric paint on a wearable fabric project...basically, you have to experiment a little, and figure out what paint to use based on the properties of the individual paints.
For instance, acrylic paint dries stiffer than fabric paint--good quality fabric paint shouldn't dry stiff at all. So for a T-shirt, or baby clothes, you def want to use superior-quality fabric paint. But for your jeans, or a hat, or any other article of clothing for which drape isn't important, knock yourself out with acrylics, if you'd like. But if you're painting even a non-wearable fabric project, but you need it to be flexible and have a lot of drape, you probably want to use fabric paint. With my peace flags, the stiffness of the paint doesn't make a difference, so it doesn't matter if I use fabric paint or acrylic.
Coverage does matter, though, and it pretty much depends on the color of the paint, and a bit on the brand, how well a particular paint will cover a particular fabric color. Even with the Jacquard Neopaque fabric paints, which are supposed to be, well, opaque, the yellow needs two coats before the blue fabric print behind it doesn't show through. Same with the green over the silvery-white fabric that I chose. The white, however, gives perfect coverage in one coat, even over the green fabric that I'm using, as do the rest of the darker colors. With the craft acrylics, every color that I've tried needs two coats before it's opaque over a dark print, so if I have to do two coats anyway, I'd rather use the cheaper acrylic paints than the waaaay more expensive fabric paints.
I like to work with what I already own, too, so for me color matters--I'd rather use the perfect color in a more expensive paint that I already own than go shopping for the perfect color in a less expensive paint. For instance, I like the green of the fabric paint much better on this particular fabric, and I thought the red of the acrylic paint gave the best contrast on the trickier-to-match red fabric, so that's what I used:
Of course, you can always purchase textile medium to give acrylic paint the draping properties of fabric paint, but that's just something else to buy.