I do have an Asus Zenfone 2. Android was never my cup of tea. I even have a Galaxy Tab tablet. It's fine and all but I find that UI to be very old school and nothing really modern about it. Sometimes it's chaotic. Speed has always been an issue but can't say Marshmallow is in the same boat.
My comparison would be better with the iphone 6. I find the iphone 6 is much quicker no doubt. Load up asphalt 8 on iphone 6 and on my 950xl and well the loading time is much longer on the 950xl. The game does look better on the 950xl and it runs pretty smoothly on both. Granted different builds produce different results. I have had builds that cause slow downs. But I'm talking about redstone which I'm currently on with the insider testing builds.
I used to like ios 8 but ios 9 really made things much worse in terms of usability. I still prefer it to android though. App quality is probably the best on ios. Now, there are unique ones that are actually much better on windows mobile. I find the twitter app to be the best on windows mobile when comparing the platforms.
Windows mobile is definitely not in the state that MS envisions it to be, that's why they are not focusing on it. When they said that, they really were saying "it's not in a state that we can give publicity to, we want it to be in a perfect state before we show it to the world". So you see updates constantly even though they said it wasn't their focus. It was just misinterpreted. Bad communication on their part. But even today a new update came out that fixes and optimizes things even more. I would stick to production builds if you want a more stable usage pattern. However, if you're into checking out how windows mobile is progressing, insider builds are great for that, but they can cause you headaches.
I think that once MS does release their reference Surface pocket phone in spring 2017, windows 10 mobile will be mature enough that reviewers won't really have much to complain about. Apps are slowly growing thanks to the windows 10 desktop focus and UWP. In a year it will be a different story. For now you are basically getting a half finished OS but a good one no doubt.
With the return policy MS stores have, I don't think it hurts to try it. I think once you get playing with the customization of the start screen you will fall in love with it. But things can only improve from this point on.
Reviewers have something called bias. You can be sure that any reviewer will be running either an android phone or iphone. They are reviewing the MS phone because it's their job, but it is being done with heavy bias. There are a few sites that do give it a fair shot. GSMarena does a brilliant review on it and definitely threw away their bias. It really shows that it's a great phone. Compare that to Gizmodo the most Apple biased site who started off their review by saying "Ugh I thought when I was being forced to use this thing for 2 weeks". That's not exactly a solid review. These people will downplay features windows mobile has, but when ios copies it, it's the best thing since sliced bread. These guys will pounce on any 1 single issue and blow it out of proportion. Which is why MS can't release their next phone with any bugs or issues. It has to be perfect.