There are a couple MU's that I personally disagree with what most Link mains say; I think Wario's 45-55, not the 40/60/35/65 some people say, and I think ROB is 35-65.
ROB's left hook (ftilt) has nasty range which hurts a lot of what Link can do in this MU (trying to space ROB). Link can't run and spam because ROB murders Link long range. Up close is lolROBadvantage. Link can be difficult for ROB to deal with if he can stay in the mid range area, outside of ROB's close range attacks and inside his safe spamming range. I find ROB's grabs to be particularly nasty against Link. Link's harder to gimp than you'd think but ROB is one of the better characters at it.
Outside of gimping, Link is going to live waaaay longer than ROB. Link with good DI doesn't flat-out die until really high but gets gimped if he's sloppy or fails at DI; if ROB doesn't die he's making it back, period, but from what I hear his momentum cancelling sucks pretty bad and doesn't really help him as much (basically they're exact opposite ends of the spectrum).
/my noobish 2 cents
When I play against ROB, I try to stay in a midrange zone outside of ROB's grab range and ftilt range but close enough he doesn't feel comfortable using his projectiles. Basically, Link can do well if he can stay in zair range (which has 2 hits, which hurts ROB a lot from what I hear), but his happy spacing zone is going to be wildly moving around whereas ROB outspaces Link almost completely outside of this tiny zone.
35:65 ROB favor imo (although I think it's worse than almost all the other Links). Stage selection greatly benefits ROB; it's more even on neutral stages but ROB can always CP a cheesy level like Frigate, Rainbow Cruise (which, 1, I'm not convinced is a great level for Link in most situations, and 2, ROB is going to enjoy the long range camping and crazy stage layout/forced moving around moreso than Link), heck, even Lylat Cruise, and Link can't ban them all.