I start from the common problems for all dwellings.
1) They have no wall and hard to protect.
2) It's very hard to improve their happines level.
3) They usually have no way to increase production speed.
4) Their units are not affected by racial happines and RG bonuses.

All these factors raise obvious question why the player should invest in them. In fact I usually don't. If I play with good alignment I prefer to keep them as vassals because their development to get T4 unit requires keeping big army in them for protection for many turns. And having 1 T4 dragon in 6-7 turns doesn't look like something that will help much to win the war. Of course there're different map settings, occasional treaures in dwelling's territory which can increase prod. speed, some dwelling have interesting strucures which increase income and production and if you play as evil you shouldn't realease settlement as vassals.

Sylvan Dwelling is very small so the chances to get any treasure site inside are low and has no T4 unit but it gives T2 unicorn really fast and in Balance Patch mod which I use unicorn has healing and first strike on elite which make them valuable enough. And T3 fairy is strong enough flyer with ranged attack. The problem is slow ploduction speed 1 unit in 3 turns while racial T3 units usually take only 2 turns (sometimes even 1 turn is possible).

CI introduced Epimeliad and Hamadryad at first. May be interesting units if you get them somehow but not something worthing heavy investment.

In prev. version Satyr branch appeared with T3 Satyr at the end. It looks not bad by the stats but In my game on this version I just ignored this branch because I can produce a lot of melee units in the cities but I've no equivalent for T3 fairy.

Now it introduces T3 Silver Unicorn. Since I love unicorns and it was my favorite unit in AoW 1 I finally was interested to look carefully and noticed the price of 170 gold and mana. Which means 4 turns for 1 unit and 6 turns to build the strucure for it. I checked other T3 units and there's the very same situation 4 turns for 1 unit except Fairy Sprite.

I think Silver Unicorn is more usefull unit than just another T3 hulk. But 6 turns for structure and 4 turn for 1 unit it's just not an option.

I suggest:
1) Reduce the price of Unicorn Preserve to 250 to match the one for T3 Fairy and the price for all T3 units to 150. I don't care about gold\mana proprtion but 4 turns for 1 T3 unit is just not an option.
2) With introduction of Silver Unicorn Epimeliad now has a competitor for a healer role. But why not to make dryads and meliads supports so they had bonuses from support upgrades or slightly increase their stats.

Another problem or question is Fairy Sprite because it sucks against any unit with more than 10 Res. I agry that Nightshade Fairy suffers against such targets too but it has only 3 damage channels and range attack. I understand that it will be devastating against anything with low Res and flanking but I expect from T3 unit to fight against other T3 and they normally have 10+ Res. But I have nothing constructive to suggest about it except obvious advices to use it only when dealing with a lot of low Res enemies like orcs and in combination with Aphrodisiaka Flowers for home guarding.