Asus are a bit of a special case when it comes to the rooting of their devices, as not all manufacturers release custom tools for this purpose. In some cases, makers of Android phones actually go quite far to discourage their users from rooting their devices.
But at least the situation is not as bad as with Apple, who are actively trying to prevent their users from rooting their phones and gaining access to the operating system’s internal components. Apple claim it’s for the users’ safety, but those who know the benefits of a “jailbroken” iPhone also know that the procedure can be very worthwhile.
As for those looking to upgrade their ZenFone 2 to a better Android release with more features – just make sure to read the release lists, and focus the search on ROMs specifically aimed at this device.
Sometimes a ROM would have special features that help it work even better with a specific device or devices by a given manufacturer, or on the other hand, the specific needs of the user. But in the general case, CyanogenMOD is always a safe option with plenty of variants that give users a wide array of choices for what they can flash to their device, and subsequently what kinds of benefits they will get.