Nuance is definitely needed here.
I wouldn't call Hayes a bust, but he is most definitely a poor hitter at this point. His .291 wOBA is well below the league average of .320 (this is actually in "awful" territory"), and his offensive contribution to WAR is *negative*. He's a phenomenal 99th percentile defender though, so his overall WAR is positive in the 1.3 range.
Hayes is really a fascinating case as a hitter. He hits the ball pretty damn hard, and he does it often. Avg exit velo is 92.2mph (91st percentile), max at 113.1 (85th). and he's got a super healthy hard hit rate of almost 48%.
In past seasons his problem was that he simply didn't hit enough balls in the air. An average launch angle of <5 degrees and groundball rate north of 50% meant he'd never get to much power in games. Makes sense right? You need to hit the ball hard AND you need to hit it in the air for it to leave the park or at least do damage in the gaps.
Now this year. Hayes continues to hit the ball hard. Hardest of his career in fact. More interesting: he increased his average launch angle to a much more ideal 13 degrees. Cut his groundball rate down to nearly 40%. These were all very exciting developments, and yet the expected power boost never materialized. What happened?
Fangraphs had a great article on this disappointment back in June:
https://blogs.fangraphs.com/kebryan-hayes-is-almost-elevating/
TL;DR -- When Hayes hits the ball in the air, it's usually *too* high and/or not sufficiently hard. The majority of his hard hit balls come with poor, low launch angles. Average launch angle can be misleading and what we really need is a measure of % hit at ideal angle with sufficient oomph.
Anyway, Hayes is still fairly young. He needs to make some tweaks and get to his power, otherwise he's looking at a career of significantly below average offensive production. A few more walks would help as well. Absolutely not out of the question that a great athlete who already does hit the ball hard can successfully make said adjustments. So the verdict is still out, although results to this point are disappointing. On the bright side, he's a truly special defender and that gives him value if he can even get his offensive production close to league average. And as others have pointed out, Hayes isn't really expensive if you know anything about how MLB contracts work. It's just that a 2-3 WAR third baseman isn't a grand slam of a signing at that price, and everyone was hoping it would be.