The transition to a new career was by no means easy, but it was ultimately rewarding.

by 在这里是介词，表示"通过某种方式"或"以某种程度"。 no 是副词，表示"不"或"没有"，用于否定后面的内容。 means 是名词，意思是"方法"或"手段"。 by no means 是一个固定短语，意思是"绝不"或"一点也不"，用来强调否定的程度。