Never looked into Civ5 mods. But like expansions, adding conceptually good things doesn't solve the inherent problems of the game in my opinion.
They can be softened up a lot though, and Vox Populi imo did a very good job of it. Civilian units behave like in 4, that is infinitely stackable with anything that isn't an enemy unit. The AI is actually reasonably clever. While happiness is still global, the effects of unhappiness are a lot more forgiving and scale appropriately rather than an instant EVERYBODY STOP EATING AND WORKING IMMEDIATELY the second it reaches -1. Instead of just population and number of cities happiness depends on a multitude of factors, and is actually quite brilliant:
Every city has certain needs, like Literacy, Safety, religious unity etc. Most of these can be summed up as "If this city produces less of a yield per capita than the global average happiness goes down, if more than average it goes up". E.g. if you have a city with 20 population and it only produces 3 research per turn (oh and research is no longer this stupid population=research formula) while most other cities produce ten times as much research with half the population this city will have lots of Illiteracy unhappiness. It works like this for culture (Boredom) and money (Poverty) too. Crime vs Safety is especially interesting in that regard, because it takes the strength of the city as a yield. That is, cities have a certain combat strength depending on buildings like walls and castles and a garrisoned unit. If you put a Tank in a city it will have a higher combat strength than if it was garrisoned by a Warrior, which translates into less unhappiness from Crime, which means you actually have a reason to put a unit or Walls into your safe inland capital. I think this is ingenious because it means you actually get a direct happiness benefit from putting a modern unit rather than just your starting warrior in a city, one that scales 1:1 with the strength of the garrison unit, something which is miles ahead of 4's military police mechanic. In fact I'd argue Vox Populi's happiness mechanic is more rewarding than 4's even. You are always on the lookout for ways to reduce unhappiness from various factors, pillaged tiles, religious disunity (which again scales very nicely because 5's religion mechanic is in some regards lightyears ahead of 4's), poverty, crime, even war weariness made a comeback.
I honestly think Vox Populi is on the same level as the likes of History Rewritten or Realism Invictus.
What exactly are the inherent problems of Civ5 anyway? 1UPT and Global Happiness? The latter has been turned completely upside down and the former is a lot more tolerable. Other flaws, like arbitrary counter intuitive penalties to expansion, a braindead and vengeful AI*, and a horrid user interface have been done away with too. Vox Populi is something between Kmod, BUG and Realism Invictus. It fixes all the broken and slow code, cleans up the UI, removes bad features and adds a ton of new ones.
I'm so convinced of Vox Populi, I'd even be willing to gift you Civ5 Complete right now if you promise me you would try it out. By right now I mean right this day, because tomorrow I'll be gone for a few days and then the current Steam sale will have ended.
*I fondly remember one game where Genghis Khan of all people kept trying to bribe me to make peace with everyone I declared war on, even though he had zero stake in any of those conflicts.