External relays

[twostar]

The built-in relays will have a longer life, especially if you use high currents for long durations. Even more so when high inrush current devices - for instance, poor quality LED lights will 'fry' the built-in relays very quickly and you will need to replace them. Speaking from experience, and wishing I knew that when I was still planning my project...

Also relays have very different current and load ratings depending on what sort of load you're switching, technically the utilisation category, of which there are a great many.  Typically if a single rating is given it'll be for AC-1 or resistive loads, which are the least awkward load to switch and so the one you can give the highest numbers for (this is why you can find pictures of melted Shelly's where someone's tried to switch something like an AC-23 inductive load at the current level quoted for an AC-1 load).  A good-quality relay will have a data sheet with ratings for different load types.

For high currents you want a contactor, which is just a relay but designed to safely switch high currents, e.g. with appropriately-plated contacts so the arcing doesn't damage them.  Alternatively, they may use the arcing to clean oxidation from the contacts.

For anything other than small loads like lights or similar I always use appropriately-rated Finder relays for the switching, not the ones on the device.

Just checked and the relay boards use Omron G2R relays which are good quality, just check the ratings if you're switching high currents or very non-resistive loads.