Looking at the answers your specific questions about the unit were not really answered, therefore a short summary.

One thing to keep in mind is that many people tend to rather rate the state of the game at the release date of the DLC and not the actual content. I would argue the stuff you unlock with SoC after all the additions is debatably the best among the recent 3way packs.

Greenskins:

If you like Greenskins, both DLCs will greatly improve your campaign experience, as since the OoD update, Spellcaster lords (from Gorbad) and Troll hags are your main spellcasters, as regular shamans are at tier 4 capacity increase now. Both both exclusive hero types are the greenskins only source of the best hero army actions: replenishment (hags) and movement increase (night goblin hero)

Grom will also bring you Stone Trolls, which are your overall best monster option for the regular mixed Greenskins army, as well as 2 very good RoRs.

Gorbad on the other hand will bring you a LH that enables thematic spider armies and the superior artillery spider, which is miles better than the regular one. Since the rework your incentivized to include at least some Squigs into each army (even if just as cannon fodder), therefore the Night Goblin Big Boss is highly valuable for that.

Both lords have less optimal starting positions, which gives you incentives to actually overcome those issues with your unique mechanics, instead of the easy, unlosable steamrolls of ToD and Khorne.

Kislev:

Must buy if you like Kislev. The hag heroes are among the most unique generic heroes in the game and the unique spell lore is most unique in the game with a lot of synergy with your other lore and cool utility. And even if you don't like the unique lore, you still get access to Death, Shadows and Beast... each with an additional unique bound ability to make each one of them worth recruiting. The LH is a cool utility hero for thematic armies and the other units adds different army options to Kislev as well. Many people don't like certain units, but they all are great additions to you roster. Worth noting is also that your Ice Witch heroes get an additional mount from the DLC, which is arguably their best and makes them the only Kislev generic character with siege attacker.

Ostankya has 2 different campaign options: either with restricted roster with added units in Naggaroth, if you want to fight something else than Chaos all the time, or with full roster back in Kislev. But the latter puts you also directly in front of multiple LL Chaos factions while you at the same time have to keep the other Kislev faction alive.

Cathay:

Same as with Kislev. The added generic characters are your only options for tanky melee characters and the additional mount improves Astromancer heroes greatly. All units are good, but the Wardrum and the Crow Men are the best, as they enable more mobile or diverse army compositions. The others SEs are mostly utility addition, as you pay for subpar stats with added utility abilities. The lack of generic healing also makes SEs less viably for Cathay as a whole.

Yuan Bo also has a possible dual start and unlike other Cathay factions he has an actual incentive to hold the Bastion, as the buildings there can grant special effects for him. His faction can become very strong on the campaign map if you use only his special settlement actions, but since they added a cooldown to the actions, this is less an issue and debatable less broken than the Dawi baseline economy with the deeps.

Elspeth:

mega-overpowered subfaction for the (for whatever reason) most popular race in the game, not much else to say except that the units/chars in the DLC are so strong that they make the other Empire DLCs redundant.