Can spyware harm your phone?
The simple answer is No; spyware and monitoring apps and services do not put your device in danger but they have some disadvantages too. First of all, they run in background all the time and use system resources like RAM and Processing Power continuously. Some apps that monitor IM activities consume extra battery; see top 10 Snapchat spy apps.
Obviously it is not recommended to buy such apps and services if you have intentions to install them on older devices because they will not work efficiently. Some advanced apps and services allow users to enable and disable different features from the web portal they provide at the time or purchase so users can turn off unwanted features. Spy and monitoring apps do not harm your phone but instead they guarantee your device’s safety because you can use features like GPS location tracking and remote access to control your device even if it gets stolen or lost.