I had a similar reaction, after a city of mine completely broke because of simulation errors I gave up on it for a while. Then revisited a bit later to write an article about all the dlcs. It felt a lot more stable then, but that might be because I did not test the edge cases again.
Biffa has some good videos on traffic solving: https://m.youtube.com/channel/UCYzO4HZxgXc2UJQD5bIo8AQ
It works somewhat realistically but you have to learn the limitations of the simulator if you want mass transit besides busses used well.
I also really liked CityPlannerPlays - quite educational but more on actual city design.
The built–in tools are also somewhat hit and miss, and that is partly driven by what information the game holds on to and what it throws away for increased efficiency. Agents know their destination and chosen route, but they don’t remember where they are coming from. Thus, when you click on a building to show the associated traffic routes it can only show routes with that building as a destination.
The exception is service buildings like fire or police; each one supplies a fixed number of vehicles, and in theory the tool could show what all of those vehicles are doing. It still just shows the routes that end at the building though, so you end up seeing just those service vehicles that are going home and people who are going to work at the service building. In principle, mods can fix some of these oversights. I use one that shows what each service vehicle is doing, for example. Another mod that I really like allows you to select a stop on a mass–transit vehicle route (a bus stop or a metro station or a ferry station or whatever) and see where everyone who is currently waiting at that stop is going. Very useful.
That said, I think Cities: Skylines is the best SimCity we’ve ever had. It is everything that SimCity 2013 should have been. I recommend it to everyone, but especially people who ever enjoyed a game of SimCity, or has enjoyed any city builder of any stripe, or who think that games are only for children.
In a real city people would just drive around that madness and make a left turn at the next block.
How single agents path is also weird, like how much they walk. I think I remember that homes were also not persistent?
When separating city blocks completely fires would not get extinguished. Either the fire station assigned is random, or only a few actually can send cars out. Which breaks down in a city not fully connected by streets.
Haven't played for a few years now (last run was with the industries dlc), but also haven't read about patches adressing such issues.