Considering putting concrete floors in bathrooms. Is concrete unsafe for inside house? Other reasons to not use concrete?

Concrete floors are used widely in residential construction. It is a light weight concrete you should use. Now you can pour it about 2 inches thick where you used to have it four inches. It will make a high transition to other existing floors so unless you want a step up into the bathroom you will need to pour in all other connected areas of the home. The transition will be the biggest problem.