The only con that I've run into so far is scorching wort in a RIMS tube, BUT I need to clarify on that with details most dont give.
One, I was step mashing with it, and two it's a 4500 watt, and three I was running reduced flow rate to prevent bed compaction, four it is much more likely to occur with high protein mashes like wheat or rye. The scorching problem does go away if I compact the grain bed and go full flow, but I didnt want to do that so I switched to a dedicated hex for mash. The input water for the HEX is heated by the rims tube, so now no mash wort ever touches a heating element.
Another con would be that sometimes the heating elements grow rust or other corrosion on them, but placing a magnesium rod in the water with direct contact to the same metal surface that the heating element threads contact solves this.
Temperature control is much more uniform and easier if you do it right with PWM + SSR and PID. That's from my observations at least.