Why they told you that it would not be a smart option?
In 8th ed you could split fire so a lascannon in an Infantry Squad can fire at a tank while the lasguns can target some near enemy infantry (dont forget that HWTs have lusguns too)
The guard is squishy so you have to avoid grouping too much
Spreading your HWT across IS gives them more survivability and forces the opponent to spread its anti infantry fire or to charge different squads.
Using Heavy Weapons Squads it is still ok as they have the andvantage that with one order you can affect 3 heavy weapons. Btw
dont rely too much on reroll orders as the 4+ to hit is not very good (but nether bad). Remember to not gropu too much in this case too, for example if you want to put 3 lascannons and 1 missile in your army make an HWS of 2 lascannon and 1 missile and put the other lascannon in an IS. So it your opponent focuses the HWS you will not loose all of your lascannons.
Mixing heavy weapons in HWS is also good because if you are not able to reach different targets the squad will still be somehow effective against any type of target.
Example: if you have 3 lascannons the opponent will try to give you "swarm" units as targets. In the same way if you have 3 heavy bolters it will give you tanks as targets.
If you, instead, will have 2 lascannons and 1 heavy bolter you will be effective against both tanks and infantry. Dont let the enemy to have any tactical advantage on you, make your army as balanced and redundant as possible so loosing any piece will not be a dramatic loss and you will still able to do the same things as before. You have to loose firepower in an even way to keep you competitive. Of course this does not apply on suicidal squads but this is another tactic that requires a bigger army and "special" units or Docrtrines.
Edited by Phubar, 15 January 2020 - 06:29 AM.