I've had organic, grass-fed, grass-finished beef that was wonderful. Tender, juicy, delicious. I also know that if it isn't done correctly, the results won't be good. You have to have a breed that does well on grass only, such as Red Angus or Scottish Highland, (I know there are other breeds well suited ti this as well). I've read that Hereford don't turn out as well grass fed. I'm just taking the word of some folks who've done it for years, it's not my own personal experience. They also recommend intensive rotational grazing, so that the cattle are moved onto fresh ground every day or two, and the growth they are moving onto is lush, young, sweet grass. Cows in a field of tall, older grasses won't turn out well.
It's just like raising meat animals by any method, if you do it wrong, your results will show it.
It also helps if the cooks know what the heck they're doing, if the meat is not the best quality. A good cook can make a tough piece of meat tender and succulent. But if raised correctly in the first place, it's no harder to prepare than corn-fed.