It's not so much the price of the
WW, but the opportunity cost of NOT taking one of the other
HS picks. In this case, the
TFC. What the
WW does is give you the equivalent of a heavy bolter and/or flamer, under the pie plate, across the table, with barrage wound allocation. It's really only useful against hordes on foot, either in the open, or skulking in the trees. And that makes it a mono-tasker. It does one thing, albeit fairly well and reasonably priced.
But if we compare it to it's peers, we see that they can do a lot more.
A Dakka pred, with the
AC/
HB, can put 8 shots into a target. It needs to hit, but doesn't scatter. So it can still shave models off of hordes, plus has the option to shoot at flyers or focus on single targets (like monsters) The S7 on the
AC also lets it take shots at light vehicles with some success.
AV 13 front armor helps keep it alive. While currently the same price, it's going up, if we assume
DA pricing, by 10 points.
The
TFC is more expensive, so we should expect it to be better. Whether 4 small blasts is going to equal one large one is dependent on a lot of things (like unit spacing and the mood of your scatter die) The fact that you are getting 4 shots will let you get multiple hits on single targets. It has higher S, but lower
AP then the
WW. And then there is the tremor shell. All you need to do is hit the target, and they are making terrain tests. This will slow them down, and give you more time to shoot. You also get the techmarine with the harness, a 75 point value. His bolster ruin will make the cannon harder to kill, and/or help some scouts get a 2+ cover save. The 60" range might be irrelevant on most tables, but it's there.
The
TFC can boost the defense of other units, hamper the abilities of your foes, and blow chunks out of things. The
WW just kills troops. As long as the
TFC exists, at any reasonable price, you need to ask yourself "Why am I not taking this for my horde killing needs" It's not that the
WW is bad, or even overcosted. But the
TFC is all that, and a bag of chips. Find the points, upgrade.