Matt Riddle really let loose on Goldberg, calling him unsafe, dangerous and a liability. He might be talking about when Goldberg tried the jackhammer and crashed the Undertaker on his head/neck area.
Now to be honest, Riddle has never been a fan of Goldberg but he really let Goldberg have it last night. And remember, Riddle is working for the WWE's brand NXT.
Bill Goldberg is also no fan of Riddle as he has blocked him on Twitter. It will not stop Riddle though and as his tweets get to the media for somehow (maybe due to their inflammatory nature) Goldberg will still hear about them.
But you are right, the match was horrendous. He should have never wrestled such an out of shape wrestler as Goldberg. Now Undertaker might not be a lot less rusty but Goldberg's last match was more than 2 years ago. And even that was a stretch IMHO, he was terrible then too.
And for those who do not know who Matt Riddle is