这句话是什么从句 还有 请分析一下句子结构 为什么用what不用which

这是一个主语从句,what is often not so obvious在整个句子里面充当主语的成分,所以不能用which.
句子结构：主语what is often not so obvious,谓语are,宾语rewards.
另外,句子中a caseworker career offers前省略了that,这是一个that 引导的定语从句,修饰宾语rewards.