I thought i just drop my two cents here having experience with KO3 and CI, so here it goes:
It is pretty hard to put Yii and Kohana in the same “basket” because the only thing being the same between them is the fact they are frameworks, but that’s all, everything else is treated in other way for each of them.
People who blame Yii(or everything pointing directly to yii) are just frustrated people not understanding what Yii means. It is also true that Yii requires more experience from a developer than other frameworks, but it also offers some amazing features and a high development speed(this is by far the best advantage YII offers).
I don’t know about you, but in the company where i work, we always have some strict deadlines and we cannot afford to go over them. Time is money, really.
I have nothing against Kohana, au contraire, i use it pretty often and i like it a lot because it is very lightweight. I also use Codeigniter for same reason, being lightweight(though is true, Kohana is taking CI place in my projects because, well… unfortunately CI is behind the times, but i am forced to stay with it because i have more than 30 active projects built in CI which needs upgrading and maintenance and further development) .
Yii makes the developer job easy enough, easier than Kohana can do it, or CI matter a fact, but this comes with a price, a higher memory usage and CPU usage for each request, therefore the environment where the app will be deployed needs to be more "special". But when APC/XCACHE comes in place, this is not a problem anymore, at least not so big.
I really think the learning curve is the decisive factor when choosing a framework. Lots of people say Yii is easy to learn, which is false, i had a very hard time understanding it and also took some time to realize what goes where and why (I am coming here having 2+ years of CI background, not counting the procedural programming years and still was pretty hard for me to get started with yii), but now i am getting the point and it makes sense to me(though i know, i have a long road before me till i will really understand this framework).
In my case, i don’t stick to ONE framework because for some projects makes more sense to use one than another, so before i start building the website i look at the requirements and at the time given till completition and make my choice between the three of them (Yii/CI/KO3)
Just as a side note, i have started a modular CMS several months ago (based on CI) which is very mature right now, but as much as i would like to rebuilt it under YII, i am still not decided if i want to do so because of the memory usage. I have in mind the issue of shared web hosting, where the servers are not very powerful(or if they are, they are very loaded anyway) and the memory/cpu limit will be always a problem. So in this case, i would rather continue with CI or rewrite it in KO3.
(P.S: i may be going too far with this memory/cpu usage thing, but i had the bad luck to face very ugly situations because of the after mentioned issues, so that’s the reason i am being this way.)
As you see, beside the learning curve, the deploying environment is another important factor to take into consideration.
In the end, it is important to choose the right tool for the right job.
Keep in mind, there’s no universal tool that will match every job you need to do (that’s why we have 50+ php frameworks and 100+ Open Source CMS systems)