Link has to space his utilt obviously. Ganon can't dsmash it for among other reasons the fact that he's pushed out of dsmash range. We explored this in training mode; all three of Link's tilts push opponents too far out if spaced to punish directly out of block, and Link has time to spotdodge running grabs (at least from DK who was our test victim; I can't promise that characters with extremely high running speed and huge grab range can't hit it but DK also couldn't dash attack any tilt on block either if Link spaced correctly).
Boomerang's start-up isn't really that long, and Link can control the angle in a way to make the powershield timing inconsistent. Even if you do powershield it, odds are you're too far away to do anything about it; Link isn't throwing a Boomerang against an opponent standing right in front of him.
Tethers snap to the ledge fast. What a lot of people don't get about edge guarding is that it's a question of time moreso than anything else, and most importantly, that time is measured from the moment the initial hit to send the victim off-stage occurs. If you hit Link off-stage softly (so he's close to tether range from the start), he just tethers and, before you can get close to the edge to prevent his recovery, he's already snapped. There's no time to gimp him in those situations. If you launch him far (which also pretty inevitably means launching up, semi-spike angles are super rare in this game), Link has time to pull a bomb before you get close and that adds a lot of layers to the situation. He can throw the bomb at you if you approach at the wrong angles (limiting your gimp opportunities), and he can exploit blowing himself up with the bomb to give himself two up specials (meaning that if you intercept the first he's not done, among other good for Link implications of that). Yeah if Link is just hanging out off-stage not ready for anything and you have a good meteor and you're already above him he's going to die, but actually creating that situation is extremely difficult if the Link player is skilled since the Link player knows that and will be always choosing the course of action to make that maximally difficult.
I don't really know about default Star Bits other than that the move is bad; if I were going to seriously run it against Link, I imagine he'd do what every character does and shield it followed by killing Luma since it's like -1 million on block. I could I suppose sometimes hit with it and do nominal damage with a follow up of "nothing"; man that move sucks why would Rosalina ever not use a custom side special...
That aside, Grav Pull isn't nearly as good here as you think. Yeah it negates arrows which are an unimportant move in this and every other match-up (I'm not sure which Link arrow type is the best to use but I'm quite sure all three are bad). If you Grav Pull bombs, you don't really negate the threat; all you do is pull the bomb toward you which is... kinda what Link wanted anyway? A lot of Link's bombs aren't even used as projectiles directly; he'll just hold them and ftilt (because that's possible in this game) or soft throw them onto the ground to act as a trap. You don't alter the timing of the explosion, and catching Link's bombs was always of fairly minor help in the first place so picking them up after a Grav Pull isn't really accomplishing anything more than other characters could do. Grav Pull can help out against the Boomerang, but Link can be tricky and angle it harshly so you'll miss the absorb and still have to deal with the return. More importantly (much more importantly really), Link's ideal footsies range is a bit too close for Grav Pull to be safe. If you absorb a projectile, Link is going to be poking at you with his sword due to the length of Grav Pull's animation. Even if you have time to block that safe ftilt, Link just killed Luma and you're in 12.5 seconds of unhappy time. If you don't have time to Grav Pull, you're blocking projectiles, and this screws with Luma a lot as it will often hit Luma (which slows him down from being able to follow you) or even worse might kill Luma outright which is your 12.5 seconds of unhappy time.
Rosalina's dash attack, down tilt, and especially grab are all great moves and all, but think about how the basic footsies plays out. Link's sword normals will almost always stuff Rosalina's normals, and challenging them is double dangerous as Rosalina since Luma can easily die out of a failed challenge. If Luma is dead Link walks all over Rosalina's neutral so you have to be really prudent about when and where you challenge stuff. Rosalina does have better frame data than Link (especially with Luma alive; Luma's frame data is really good), she especially can project a threat while mobile better than Link (since she has a good running grab and a good dash attack while Link is poor in both areas), and her juggle game is very effective against Link (you sometimes have to watch for dair, but it's not too hard to deal with). Rosalina's physics also help her escape combos from jab which is a huge help; if Rosalina weren't light and floaty Link would definitely be a counter so it's a good thing she is that way (I can't see how her being light is helping Link here at all). Of course, Rosalina often kinda stalls out on getting kills especially against heavy characters who don't jump much (like Link) whereas Link scores kills so easily by just hitting so hard.
When it all hashes out though, the match-up is just a footsies war. Link's pokes win priority wars, but Rosa's pokes win speed wars. Link gets a bigger direct pay-off from hitting, but Rosa has better follow-ups. Link has a much superior projectile game, but Rosalina can threaten while moving far more effectively. Rosalina can chase Link better than Link can chase Rosalina, but in closer pressure situations, Link has a lot of tools to protect himself (the utilt that hits everywhere, a good conventional nair, bombs) while Rosalina's normals are very bad at "get off me" and Luma dying is a common second penalty to her which is a type of problem Link never has. It's probably a bit easier for Rosalina to do damage, but it's at the same time easier for Link to convert damage already done into kills. Everything in this match-up has two sides and is a trade-off; maybe as the game develops things will change, but based on the game as we understand it now, how is this one not even?
Also, to be clear, I usually beat the local Link main since I'm a stronger player than he is (he's kinda new, only been at it for a few months), but the potential of his character is very obvious to me the more I play against it and the more I see him learn all the great things Link can do.