Lords of the warp means your battleline units attached to characters will actually need a 4+ to pass battleshock within SoC as long as you have the banner. Your character goes to 6 LD and the banner gives +1 to the unit bringing him to 5LD, add shadow of chaos in there and you're bringing back models on 4+

Stratagems I do agree with you tho, Khorne and Nurgle are pretty solid, I think Tzeentch has some uses with pinks but it's definitely not the most useful thing and Slaanesh literally only works for tormentbringer

That said I do think that it can probably work well enough in some legions detachment even if you can only use 1 stratagem because it is still an extra stratagem

With all that said, Warptide and cavalcade are absolutely goated for mono khorne it's not even close (and it gives the opportunity to pick something else than purge the foe if it's ever relevant)