As said before. ngSpice (which is what is used in KiCad) is a pretty decent simulator, and the integration with KiCad also works quite well.
It is a plus that you can use one schematic both for simulation, and for designing the PCB, but there usually are significant differences. For example, for simulation, you use a few ideal voltage sources (batteries), while on the PCB you may have an SMPS circuit, and you don't want to simulate that at all.
I've used KiCad for some simple projects for quite a long time (7+ years), but have never done much with any kind of spice (Well, I read Dune).
There are a lot of models that do not work in ngSpice. Some do not work because of some kind of encryption. Other models rely on extensions which are only available in some specific simulator.
There are hardly any spice models installed by default with KiCad / ngSpice. Apart from the reasons already mentioned. Many spice models are copyrighted. For KiCad itself, this means that any spice model that does not have an explicit license that state that it is free to distribute and use, can not be distributed together with KiCad itself.
A handful of months ago there was a complete re-design of all Icons used in KiCad, and the main reason was that the origin of a part of these icons was not known, and apparently some had at one time been copied from some other project with no paper trail (Think of generic icons for "file open", "file save", "copy" , "Paste", etc. KiCad-nightly V5.99 now has this completely new icon set, and a common style throughout the whole project.
So if you want to use KiCad with ngSpice, you have to cobble together your own set of spice models for now, and you also have to match the spice symbols with the schematic symbols. Pin numbering of opamps for example can be different between the schematic (which uses real pin numbers) and and the spice models. Take any quad opamp. It's 4 times the same spice model, but 4 different sets of pin numbers in KiCad, or you can't use the same schematic for the PCB.
Many spice models can be found on the 'web, apparently there is (was?) a Yahoo group related to ngSpice models, but I never used it.
Finding the models, and mapping them to KiCad schematic symbols is quite an extra burden if you're new with simulation. Your first simulation is almost guaranteed to **NOT** work, and then you have to go figure out where the problem is.
As I said before, I have not done much with spice, but the KiCad forum is frequently visited by some people who are knowledgeable in this area, and questions about ngSpice in KiCad are answered adequately over there.
https://forum.kicad.info/