A little while back during (I think) one of the codex reviews, one of the commenters had what I thought was a pretty good idea: take a look at various units that are bad, overpriced, underwhelming, or simply not very useful and explain why they are regarded this way in a bit more detail than we might otherwise go into to help give an insight into what makes a unit work- or fail to work- within the context of the game and the various books. It struck me as a rather clever idea, so I'm going to go through and review some of them by ones and twos.
I'm starting with a fairly arbitrary choice from SM's Heavy Support slot, but this won't be particularly organized by codex or any other factor. Not all of the choices will be terrible or even particularly bad- many of them will be the more borderline cases, units that are either on the edge of sometimes being usable or that appear acceptable at a first glance, but find themselves bogged down with a variety of less-obvious problems that make them unacceptable. Doubtless we will see plenty of angry replies about how many of these units are perfectly fine and "you just need to know how to use them," but, well... internets
The Good
Looking at the Thunderfire Cannon's upsides, they are not-inconsiderable; it isn't too expensive, clocking in at 100pts, and comes with a Techmarine, who can do all of the usual jazz if he gun gets destroyed before he dies. He also can bolster a ruin if one happens to lie in our deployment zone, which can be useful for the more defensively-oriented SM codex. The TFC itself puts out quite a lot of firepower between its different fire modes; all of them net you four shots per turn with small blasts, which can do some major damage to units that are clumped up due to terrain or being recently disembarked.
Surface shots are S6 and AP5, which is pretty acceptable against Marines (wounding on 2s) and can threaten most vehicles to a limited degree as well. Airburst shots are only S5 but ignore cover saves, giving you the best damage against Guardsmen and the like, though they often won't ignore armor saves. Subterranean shots go all the way down to S4, but force a Difficult Terrain test on the unit when it moves next, which can be helpful when looking to slow an enemy advance.
All in all, the TFC's damage output is its main strength- a hail of small blasts will average ~7 hits against units that aren't excessively clumped and generally wounds on 2s against whatever target you choose to fire on. It may not be very good against vehicles, but it at least can hurt them (or at least the transports) and the included Techmarine can have some nice side benefits. The initial appearance is of a fairly decent utility platform in a slot that doesn't have very many good choices, all things considered.
The Bad
There are some real caveats to all of the above advantages, however. First and foremost is the deal-breaker: survivability. With a T4 character and an AV10 vehicle that dies on any damage result, the TFC is very, very fragile. Now, the Techmarine has 2+ armor and the Cannon will benefit from a cover save as normal, but it only takes a single failed save to render the whole thing useless, and since the artillery rules dictate that most of the hits will go on the Cannon (the more fragile of the two), any S5+ shots will be very dangerous to our unit. In fact, the S6/7 torrent guns that abound in the current environment- Scatter Lasers, Autocannons, Psycannons, Multilasers, etc- are a huge threat to the Thunderfire Cannon, as it lacks the usual protection of the vehicle damage table that make standard tanks so hard to kill.
Compounding this vulnerability are several others; since it follows the artillery rules, the gun is all but immobile; it can't move and shoot and all difficult terrain has a chance of destroying the gun as per dangerous terrain- and remember, we NEED to be getting a cover save or the shoddy AV10 will doom us from the very beginning. Now, this alone isn't damning- many other units are less than mobile and still see use, and the SM codex in general isn't about zipping around the field. However, it does mean that any kind of mobile/backfield threat can pose a severe problem for us, since we can't really get away from them and, unlike tanks, we can be tied up in melee and kicked to death pretty easily.
We also have to consider positioning when looking at the TFC- because it is more or less locked into a single locale when deploying, we need to find a spot that offers clear lines of sight to any targets we want to hit (the gun isn't barrage) but that gives us as much cover as possible. Finding such a prime spot on a board will be a rarity- more often we will have our options seriously limited by the available terrain, and if the enemy goes second or has good mobility, we may be denied shots for several turns or even the entire game.
The Competition
The nail in the TFC's coffin, however, are the other options in the HS slot. Most of SM's choices in that spot are pretty mediocre, but there are a couple standouts- the Predator (in its AC/HB configuration) and Rifleman Dread (if we take a Master of the Forge for HQ). Both of these units offer us a deal similar to the TFC- good long-range firepower for a reasonably cheap price, but both of them are more mobile and more survivable than the TFC is.
The Predator is the main competitor, since it doesn't require a specific HQ and solves the TFC's two main difficulties. Although hardly what we would call a paragon of mobility, the ability to move and still shoot one gun puts it worlds ahead of the TFC (which can't move and shoot and risks destruction, rather than immobilization, on a failed terrain test)- and in an emergency it can move 12" just like any tank and even tank shock. Survivability is the real boon, however, AV13 on the front makes the Predator a tough customer to inflict any kind of damage on, much less something lasting, and its trio of weapons leave it resistant to just having one gun stripped off and ignored. Side AV11 is lower than we'd like, but its range makes taking advantage of this relatively difficult (as opposed to the Vindicator, for example), so we can't mark it as excessively limiting. Though the Dakka Pred can't boast the TFC's huge upside damage output on a disembarked squad, it boasts more than reasonable firepower (4-5 wounds against almost any target) and a similar ability to threaten light armor.
Conclusions
With the Dakka Pred being cheaper, more survivable, more mobile, able to block for other tanks, and having only a little bit lower damage output, it is hard to justify the presence of a Thunderfire Cannon in a list- the Pred is simply superior on too many levels. This is the fate of many of the units I'll talk about in these reviews and the reason for the quotes around the word bad in the title- many of them are not particularly horrible per se, they just don't stack up well against their competition in some way or another. This isn't to say they aren't usable or that you can't ever do well with them- some lists can make use of their advantages and mitigate their disadvantages enough that they will perform well enough. However, these lists are the exception, not the rule, and most often said lists will have issues of their own, as they will be rather "non-standard" for their codex and may not take advantage of its strengths, etc.
Only a very small class of units are what we would call ultrabad- units with absolutely no uses at all in anything resembling a list that wants to try and win. Most units, rather, fall into this middle ground of having some strengths, but not enough to really justify the hoops that must be jumped through in order to make them work. For these "bad" units there are often many, many defenders who will point out that their chosen unit has plenty of uses, which is true in its own way, but it ignores the fact that there often exist other units with fewer downsides (or ones more suited to the codex at large) or more upsides.
