What dresses she has bought are expensive.

= All the dresses that she has bought are expensive.

What dress she has bought is expensive.

= The dress that she has bought is expensive.

从以上二句对比可以看出，虽然what从句在形式上是个名词性从句做主语，但本质上或者说语义上该从句是个被限制性定语从句修饰的名词词组，dress的单数或复数形式，决定主句谓语动词的数。

所以名词性关系分句先名词词组一样，不能采用外置结构，即不能用it作形式主语或形式宾语而将what从句外置。例如：

Is it expensive what dress she has bought. (X)

另外，和名词词组作主语一样，名词性关系分词作主语的句子，可以直接通过倒装主句的操作词而构成一般疑问句，例如：

Are what dresses she has bought expensive?

而其它类型的名词性从句作主语的句子需要倒装时，必须借助形式主语it，主语从句外置。例如：

Where Covid-19 virus comes from is not clear.

Is it clear where Covid-19 virus comes from.

不能说：Is where Covid-19 virus comes from not clear? (X)

所以，融合型关系代词或关系限定词引导的名词性关系分句在形式上属于名词性分句，但也要认识到本质上这种从句是一个名词词组，不同于其他名词性从句。