I thought they were able to take the exact same heavy weapons options as imperial guard, what with the kits being interchangeable.
... except they can also take mining equipment without sacrificing a 2nd man to carry binoculars.
They get the same Heavy weapon team options, but they can't take plasma guns or meltaguns, which are two really,
really nice options for guard. In exchange they get web guns, which are meh. Wound on S and have decent AP small blasts, tiny range.
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.
Also no First Rank Fire! Second Rank Fire or Bring it Down! is a big hit compared to a guard platoon. Guard can pour out a lot more fire than Neophytes, Neophytes just get to appear and vanish all over the place like the plainclothes rebels they are.