While all of your soldiers can easily carry any grenades they want into battle, Grenadiers often specialize in raining down explosive destruction on the battlefield, and accordingly can easily carry on more with them than anyone else. Propelled by a specialized launcher, their grenades fly further and hit for more severe damage over a wider radius. Because so many have the cover-providing environmental elements in XCOM 2 are destructible and explosives can shred armor, Grenadiers serve primarily as a control class, reshaping the battlefield in your favor.
The Demolitions Expert skill tree mainly focuses on the former, with skills like Heavy Ordinance, Volatile Mix, and Salvo allowing for grenade fire to be more frequent and effective. Given how much AoE damage and environmental destruction this basic skill package provides, Suppression and Rupture from the opposing tree stand out as particularly useful over their somewhat redundant counterparts in Demolition and Saturation Fire. EXO/WAR Suits from the Proving Ground are particularly effective, as the additional armor stacks with Blast Padding to make Grenadiers into tough tanks, and the suit-mounted heavy weapons benefit from bonuses like Salvo.
Heavy Gunner, on the other hand, emphasizes the cannon and shredding armor through direct fire. Shredder, Holo Targeting, and Chain Shot can easily wreak havoc on the highly armored robotic enemies encountered later in the game, so you need to start ripping through armor and then quickly setting up fellow squad mates to finish the job fast. And Rupture do remains a key skill, too, especially for the huge enemies like Sect pods. Salvo from Demolitions Expert can be more tactically disruptive than the guaranteed hit from Hail of Bullets, so it’s often better to compensate for the Grenadier’s diminished aim through weapon modules and PCS upgrades instead. Autoloaders and Expanded Magazines also help compensate for the aggressive ammo expenditure of the Heavy Gunner‘s more powerful abilities.
All of the Grenadier’s grenade-related bonuses really apply to all grenades, such as the defensive Smoke Grenade or the various Experimental Grenades from the Proving Grounds. Armor-melting Acid Grenades are often particularly synergistic with what Grenadiers are already trying hard to do.