Default [Q] Custom Lockers

I'm the type of person that loves customization, but only if it looks professional. I was wondering if custom lockers that are available from the play store are legitimate or are they just "apps". By that I mean, if I'm using a custom one, will there ever be a time where a lack of ram can cause it to bug out like not opening, are they going to show my homescreen for a second before running, ect? If so, what makes them different from the stock lock screen? Is the stock locker an app? Is there any way to make a lock screen part of the system? I am rooted and I'm using stock rom, mainly because my phone, ZTE N9810, has no development on it.