There is always the reasonable chance the units are duds (physically not reliable) so it’s a process of elimination of try everything else first.
So let’s eliminate logically.
When you say it’s unstable, the chances all the hardware is unreliable is less than if one unit is dud. So is there a pattern to the unreliability? Is it that all the units (all cameras etc) lose connection? If so then it’s pointing to the Homebase being the fault.
If it’s just say one camera then it’s probably that camera fault. Try swapping the cameras placement and see if the fault stays with the camera or stays with the location. If it follows the camera then you found the dud at that camera. If it stays with the location then the fault is caused by location and then use standard techniques to eliminate interference.
If you do find the fault as per the logical root cause analysis method, and it’s a unit from Eufy, then you have accumulated the proof and much harder for Eufy then duck doing the right thing of replacement.
Based on what you’ve written, you state the camera nearest to homebase is least reliable, so therefore swap the cameras and see if the fault follows the camera or sticks with the location. If location then try modest movements of the camera, even a few inches movement can help avoid whatevers is the cause.
But also as you said eventually all units have to be resynchronize, makes me think it’s the homebase at fault. Try moving the homebase therefore.
Once you done that, and still issues, make a warranty claim and swap the parts.
An example of what could be not a fault with your units but does affect your units is a sustained electromagnetic pulse which knocks them all out of sync. Examples would be someone nearby has a leaky microwave oven (bad door seal) and everyone they use it for a long time everything electronic nearby goes wonky. Or you’re in the path of microwave transmitter, or a bad Radar (near airport), etc.