You're right, but the latest MTKs are good SoC... The screen isn't very good but it's better than Xperia E3's screen. Last but not least, Sony is a well-known OEM instead of THL. Amazon.de sells Xperia E4g at 119€ (preorder).
This is as true as stability is (also) a driver matter. KitKat is a little buggy with videos because of drivers, I can think the same about Lollipop and I'm worried about new bugs due to drivers.
Our wonderful devs are still human, I don't think they can make miracles :P
KK is smooth enough, with little bugs and sporadic crashes. It's muuuuuch better than stock firmware, which is a little more stable but too slow, ugly and useless. However, I advise you to buy another newer phone, because Y300 was launched almost 2 years ago... Just have a look to the market :)