Ynnari are built around Infantry and Bikers as those are the only ones benefiting from SfD. Minimum size units, packed tightly together, move as one or in 2-3 blocks, compensating for the loss of 1st turn with Soulburst.
OFC with split fire in 8th edition a clever oponnent will never really kill your unit until later when all potential soulbursters are limited in output.
Therefore units like 2 khymerae, sacrificing themselves to trigger the burst are needed IMO.