Actually an ambush is going to be stronger, the greater the dps you can level on the enemy before they retreat. For this reason, the stormtroopers are better at it, which isnt to say pgrens are bad at doing it but you want to front load your dps during an ambush, not trade fire at medium range.
So what you are saying is that currently since pios are underperforming for cost, there is no overlap but if pios were more powerful there would be?
I have already strongly advocated for pios to be cheaper, which would make the more cost efficient. This will also prevent your supposed overlap.
Grenades are meh against buildings because the unit inside can pop out and back in. This is a very basic micro trick. It does not work against incendiary grenades or molotov because they deal damage over time.
Wiping a squad before they retreat is not a good thing. That is why many AoE systems are designed not kill everyone, just wound the rest. Bad positioning and clums usually cause the squad wipes.
Pioneers should not be unreasonably held back. This is just creating a problem to be "fixed" later. Pioneers need a damage buff as long as there is no T0 Grens.
Except incendiary grenades are building specific. One can hope out and hope back in and still avoid most-all damage because it still only effect models in its radius. Many two store building are tall enough where it second story units are not damage by the pool.

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