Well, how about we look at what each chapter brought, shall we?
Dwarves – the first guest race and pretty easy, just slow (especially at first, due to the Granite mine's production)
Fairies – the second guest race, and the first where their buildings were connected to each other – you needed stuff from Day Farm for some Night Farm productions, and stuff from the Night Farm for some Day Farm productions. You also needed to upgrade your portal before you could even store your capped resources
Orcs – a very slow chapter, unless you dedicated a lot of space to Mushroom Farms and Rally Points. Introduces the Orcs – a new resource that is needed for upgrading workshops, for negotiation/catering, and for training two new units. Also has the most useless Culture building – the Campfire BBQ. It is at the end of chapter and is quickly made obsolete by the Weeping Willow
Woodelves – they introduced Mana (the first decaying resource) and Culture buildings that require a road connection. This is also when the new city street we research stops costing any guest race resources and instead costs only Coins, Supplies, and Mana. And this is also the chapter when Main Hall upgrades start requiring Mana
Sorcerers and Dragons – the main challenge here was space, due to the lack of streets and the size of guest race buildings. The chapter also required a lot of Mana and you needed to put some thought when deciding what to produce and when
Halflings – this chapter introduces Divine Seeds (the second decaying resource) and the first Trader upgrade. It also encourages people to go and scout as much as possible, because more completed provinces means more Seeds (and also improves the effect of a number of AWs). Rather slow-going at times
Elementals – the second Trader upgrade and the first Sentient Goods. The Canals were a bit different from the usual guest race roads, as they had fixed shapes and were producing a settlement resource
Amuni – the challenge here was reorganizing your city because Goldsmiths had to be next to residences and Trap Makers had to be next to Workshops. It also puts an extra limitation on what you can build by using Prestige to limit the number of Goldsmiths and Trap Makers. If you followed the story quests you didn't get much of a choice, as you needed 8 Goldsmiths and 4 Trap Makers, each at level 4 – and that is the exact max allowed by the max Prestige you could have. It could be a bit slow-going at times. Oh, and more Sentient Goods and another Trader upgrade
Constructs – yet more Sentient Goods and another Trader upgrade (seems to be the last one for a while). The settlement buildings are a set, so you can improve the production of certain goods by rearranging your buildings. Not sure what else they add
As for the units, all I can say is: We're almost out of units to upgrade. Next chapter we should get the Ranger, Valorian Guard and Frog upgrades. What then? With all units having reached 3 stars where do we go?