Love mine. You're right: if part of it fails, you'll have to buy a new cagdeck. Or have it repaired of course. Or have a new deck sewn to the cag. Won't be cheap I guess. But anyway, they typically wear before they fail (both the cag and the deck part), so by the time they do fail, I trash the whole thing

But since I don't use them as my only cag and deck (see below), it normally takes many years before I have to trash them.
Like I said, I love mine. Mostly for being dry, and another advantage is that they have a loose fit around the belly: easy to move, easy to breath.
Main disadvantage: you're not as flexible. E.g. I have boats with very different cockpit sizes, and my cagdeck only fits on the ones with a large cockpit. Same with the cag: you can't just switch from long to short sleeves if it gets warmer...
So to summarize, they are great, but you probably don't want your only cag and deck to be a cagdeck