While the CP cost of detachments (rather than a CP gain as last edition) changes the calculation somewhat, I'm not sure its enough to actually make summoning the better option.
Benefits of summoning vs. detachment
1. Doesn't cost CP.
2. Flexibility of being able to bring onto the unit you need, rather than having to decide before the game.
3. Units to be summoned can't be targeted until they come onto the table, and don't cost CP to hold them off the table.
4. For armies that have such a rule, summoning demons doesn't take away any rules that require your entire army to be from the same codex (which is only relevant, I think, for Thousand Suns and Death Guard - CSM don't have any such rules yet).
Drawbacks of summoning vs. detachment
1. Summoning is tied to your characters, which means that if the character isn't on the table (as reserves or being dead), you can't summon.
2. Summoning is tied to your characters but still has to follow the 9 inch rule for deep striking, which may limit where you can bring them in (there may not be enough space near your character where you want the demons to come in, or your character may not be where you want the demons.
3. Summoning is only possible if your character stays still.
4. Summoning can fail.
5. Summoning can injure/kill your character.
6. Units that aren't summoned yet can't draw fire or act as screens.
7. Summoned demons can't benefit from relics, stratagems or loci from the demon codex, as all require a demon detachment.
8. Most demon units are best in melee, and a 9" charge without strategems and with the new reroll rule (most reroll both dice of charge roll) is difficult.
Personally, for me #3 on the draw backs is still the biggest reason not to summon demons - generally you want your characters to be mobile and requiring them to stay in one place for however many turns you want to summon is a huge drawback. That said, I think if you have a really good understanding of what the capabilities and strengths are of the various demon units and the holes they fill in your army list, I think a skilled player can get a real advantage from using summoning - certainly I've read about tourney armies using summoning to succeed, but I don't think I have the knowledge or skill to do so myself.
And yes, there is also the question of whether mixing demons in with your marines is worth it at all. I haven't really given much thought of that, though I suspect there are some lists that would benefit from being able to include some of the best demon units (for example, a melee emperor's children list might benefit from bringing in some of the better slanneshi demon units like keeper of secrets and fiends) even at a CP cost.
Edited by Dr_Ruminahui, 10 May 2022 - 05:12 PM.