As far as flamers go, I think the number able to be taken in a squad as well as the mechanic for rolling for them are two factors we don't yet know the details of.
If you can take, say, a flamer as a free upgrade over a lasgun in any guardian squad*? And what if the total number of hits is Nd6 where N = the number of flamers in the squad, vs rolling for each flamer? If you take 10 flamers, and then roll 10 dice and add them up.. statistically you get much closer to the mean. Two things we simply don't know right now but could make a difference in the effectiveness of the weapon.
Overall, I am very much enjoying the direction the game is going, and am ecstatic I took the time and effort to magnetize my newly acquired wraithknight
*Please note, that is a hypothetical and not something I see as actually happening - just pointing out there is a bit we don't know yet