What is the volts for VIN? Might use pullup resistors for I2C, What volts does the relay coil use?
I see that the power is daisy chained from unit to unit. I am guessing that ground is also daisy chained. Daisy chaining ground and power is a BBBBB AAAAA DDDD idea.
As far as I can zoom in those solder joints do not look good. but if its "working", shrug.