No Sonoff device is natively compatible with SmartThings. You need to use one of the various DTHs available. My personal favorite right now is the Sonoff DTH ([RELEASE] Sonoff, Sonoff TH, S20, Dual, 4CH, POW, & Touch Device Handler & SmartApp ($5 & $10 Smart Switches)). If you are running Tasmota on your Sonoff (because why WOULDN’T one run Tasmota?!) I use https://github.com/BrettSheleski/SmartThingsPublic/tree/master/devicetypes/brettsheleski/sonoff-tasmota.src.
Now, if you DO stick with Ewelink, then yes, IFTTT integration will allow you to do various things with your Sonoff. But, why go cloud when the whole point of getting the Sonoff is to flash it to run locally? 