I've installed Opera Browser and Opera Mini (both for Android) and it seems that Opera Browser builtin adblocker doesn't work, even though it's already activated in Data Saving menu. Opera Mini adblocker, on the other hand, works normally.
Another weird thing: the adopted memory has a better performance than internal (random I/O included). I'm guessing that is true for almost all Chinese phones.
That depends on cache size too. Currently running Opera 37 beta (old UI) on (low memory) Android 4.4 the cache size is about 70MB and after that doesn't have a 'readable' location instead could be erased very fast.
Nothing that can be done to get it to install on Android?
From what I know could be a device issue not necessarily connected with Android version. I've seen the same problem on a Chinese iPad clone (Intel inside) under Allview brand running Android 4.4 non-certified by Google. Perhaps your device is too new or ain't tested or in any case is not in Opera's database and so on..