When I first saw Tremor, I thought it was borderline op, but in reality it's pretty difficult to deploy offensively...it's expensive, cumbersome, if you have it "fire at will" it will often do as much damage to you as to your enemy...against T2 units, it needs to fire a long time to do signifant damage, and enemy can usually kill it easily before it happens. Compare it to only slightly more expensive Vanguard (which admittably is T3 unit), and Tremor comes across as poor value.
On maps with chokepoints, 3 Tremors set on Hold Position and firing at 3 close points will close that chokepoint, for cheaper than any other combination of ground forces, and beat the heck out've static defenses of the same costs.
If you're using it offensively to try to defeat bases directly, I think you're missing the point of the unit, tbh. It's basically a Guardian that can move, and fires mortar-style. You can fire it over a hill, and expect it to work just fine. And if you mask it from radar, it'll be hard to perform counterbattery against, because it could be almost anywhere- that's one of the hidden advantages of the inaccuracy.
People seem to think they're worthless, and I have yet to see anybody use one effectively in a replay... but I think that's because nobody has considered that when you get to 3, you're not talking about maybe 1 or 2 hits per unit in an attacking force, but 5-6- enough to kill a lot of things outright, or wound them enough that you can very cheaply snuff them with other stuff. Front with cheap, disposable AA in streams, and you're good against a wide variety of attacks, for a cost-over-time of practically zero, compared to the costs of dislodging them (do the math, figuring 5-6 hits per unit on an assault force, or the amount of air you'd require to kill three Tremors guarded by an endless stream of cheap AA Kbots... it's pretty funny).
Personally, I think that on bigger maps with the right terrain, it's one of the more frightening things in BA, because you can effectively tower-creep with it- just cover them against Air, and bring along a couple of units that can snipe any survivors. Just don't build one and expect it to work- like any saturation artillery, it requires that you get enough firepower that hits are certainties, not possibles. If you're playing on a small map, Tremors are right out, due to the speed at which contact and play will occur, if people are any good, but on larger maps, I think they're far more effective than any of the other mobile arty, for the task of area-denial. In FFA on some maps, you could pretty much hold a flank with them and AA, especially if you can mask them behind suitable terrain.