Any issues are likely to be driver related. A major software gremlin in the Galaxy S2 chipset drivers were diagnosed in Lollipop which were present but less frequently triggered in KitKat. It caused S2 devices to hang, run slow and lockup. The same driver (for Exynos chipsets) didn't affect those devices... There were also WiFi issues but these are often resolved by formatting the data and system partitions before installing Lollipop.
It impresses me that clever and dedicated software engineers in and on the periphery of the CyanogenMod community can bring the latest version of Lollipop to a device almost 5 years old. As a consequence my battered and cracked S2 is running a more up to date version of Lollipop than my Galaxy S5!