The mining lasers are pretty nice, half range Lascannons for 5-10 points less. Would be useless without Infiltrate/Cult Ambush though since the Neophytes can't move and shoot them without Snapshots. Seismic Cannons are even more meh, it's a lot of points to spend on a gun that plinks off of meq forces unless you're in 12" range, and Neophytes don't want to be that close to marines.
Outside their detachment Neophytes for babysitting backfield objectives are best off sacrificing two guys to make an Autocannon team, because at least that has range, but when you start looking at Cult Ambush, or Infiltrate from their detachment, then Mining Lasers blow the other gun options out of the water. Would still rather have meltaguns myself though, which is why I prefer Acolytes, since they can take 'totally-not-chainfists' and rip vehicles apart in combat and have Rending on their normal assault attacks.
If mining lasers are heavy, then wouldn't appearing with cult ambush count as having moved?
Also I feel like Seismic Cannons are for "rending" vehicles from far away. Use the S5 and aim for 6's on A11 transports in order to ramp up that explosion chance.
Cult Ambush counts as having moved on some results but not others. Results 1 and 2 have you move on from the board edge, so you'd count as moving, the others use the words 'Set up the ambushing unit' and the intent appears to have been that the unit is stationary for all intents and purposes except they can't move after setting up, with the idea being that they had been lying in wait until the enemy walked into their crosshairs. Result 5 even gives a bonus round of shooting attacks for the unit with Pinning on all the attacks.
Seismic Cannons are supposed to be short range elite infantry mulchers. They fire 4/2 shots depending on range, and under 12" they're S8 AP3. They're one of those rare weapons where one fire mode isn't actually better than the other. 12-24" they mulch anything with a 4+ or worse save with 4 shots each, 0-12" they double out anything with toughness 4 or lower and blow through 3+ saves, but only get 2 shots.
Mining Lasers are 24" Lascannons. Literally identical otherwise, Heavy Mining Lasers from the Goliath truck variants are 36" instead.
Then you get web guns, which have AP= enemy S. So it's AP3 against guardsmen or 'gaunts, AP4 against marines and so on. Overkill on geq, useless on meq, great against Craftworld Eldar of all things.
The thing guard should be jealous of is Goliaths really. Orks, Chaos Marines and Eldar would all love them, and while they wouldn't benefit guard as much they're still one of the best transports in the game, though Chimera are probably better for an army with as weak a melee game as guard, FA12 and two Heavy Flamers is pretty spoopy after all.
A Goliath is 50 points for what is basically an open topped Rhino with a Heavy Stubber and a twin-linked Autocannon, a 4+ save to ignore the effects of Crew Stunned, Crew Shaken and Immobilised, and it can pay 10 points to let one passenger throw S8 AP2 large blasts from the back. 8" range, so it'll probably blow itself up pretty fast, but throwing dynamite at people from the back of a truck is pretty damn metal. If more armies could take these every non-bulky assault unit would be cramming into these. Alas the only assault units that get them are Acolytes, Metamorphs and Aberrants.