Specifically within Codex: SM. Are they ever worth it? On the units that you'd traditionally want to field assault cannons (Terminators, Dreadnoughts, Land Speeders) they are expensive and/or outshined by other options (cyclones, multimeltas and Tornado configurations). You can also place assault cannons on some other units but I'm not very familiar with them. Do assault cannons still have a place or is it generally better to give them a miss in favour of other options?
You're mostly right. Land speeders, because of their speed, tend to fare much better with a Heavy Flamer and a MM. The range isn't a problem and it can deal with almost anything it encounters.
Terminators love cyclones as their range and 2 shots make them very attractive choices. The only time you would want an asscan over the cyclone is if you plan on deepstriking them. But even then, you pretty much have to do this every game, whereas the cyclone gives you the flexibillity not to.
On dreadnoughts however, I actually find the asscan quite useful. Because most dreadnoughts tend to get into the thick of it, either by podding down or simply waltzing across the board, only the rifleman dread seems to never move. And here, the asscan provides some lovely flexibility over, say the MM.
First and foremost because it is more useful against the widest array of targets. Podded down behind a leman russ? Does it really matter whether you fire a MM or an asscan? It gonna hurt anyway. Inbound hammernators? You prefer the cannon don't you? Swarms of pesky orks? Still the asscan.
When podding, the asscan may not be as godly as a MM to take out tanks. But it gets the job done.
And when your dread have to walk the whole way, the 24 range of the cannon really helps out (because the MM have to be within 12 to actually be better than rocket launcher).
In short, a dread with an asscan is like a swiss army knife. Sometimes you'd rather have a real screwdriver handy but more often than not you can do without. At least give it a try and see what you think.