Prokofiev's first may be a trifle, but it is not bad by any means. That's what he meant.
I saw Brahms 1st mentioned. That seems too silly to reply to. Clearly just a provocation. I also saw Schumann 1. Except, IIRC, Schumann 4 was written before Schumann 1. I like both a lot.
Shostakovich 1st. I'm sure it's bad, I've never heard it. I've never gotten it on an individual disc and the notes of every Shostakovich cycle I have warn me off it. Must be really bad. 
If we ever nominate a thread for worst last symphony, I nominate Nielsen.
Nielsen has a good case for worst last and best first. (Though my own personal "Best First" rankings would probably go Berlioz, Walton, Prokofiev, Schumann, Nielsen.)
This is going to be a super unpopular opinion but...I do think Brahms' First is at best "problematic." All that drama in the first movement, and then the movement ends happily and resolves itself into peace, followed by 15 minutes of peaceful calm and cheer, followed by a heroic finale completely out of proportion to the bucolic simplicity of the previous two movements. Of course, Mahler did the same thing in some of his symphonies, having super dramatic beginnings and endings with a whole lot of cheery pastoral fluff in between. It leaves me......well, not
cold, I like all those pieces, but it does leave me with mixed feelings.