As already stated numerous times before me, there are people who are successful with tap water and with RO water, reef or FOWLR. This is one of my favourite
tap water Mixed Reef tanks. Mind you, he has recently switched over to using RO but was very successful with tap water for a long time and never had any issues with tap water in his reef (that I'm aware of).
So yes, you can use tap water successfully so long as you are doing everything else right
The one big downside to using tap water though is its consistency as was mentioned. Ultimately if you use tap water you don't really know what is going into your tank. Whereas if you use RO, you know that it is water, period (so long as your TDS is zero). I think this is the biggest reason for using RO.