I'm inclined to think that Rosa has slight advantage against Ryu in both scenarios.
Ryu has his trademark damage and killpower, but struggles to approach.
Just from that statement alone, he's more or less a Luigi with a less-spamable projectile.
Tatsu's really do tear Luma up, though, and are likely to send him tumbling to death, but Rosa can easily block and punish, if not outmaneuver the tatsu altogether, potentially saving Luma as well.
Rosa's aerial characteristics prevent her from being forced to take too much crap with Ryu's soft tilts, but his hard tilts will hurt.
Also, Ryu's DownB will largely be useless for multiple reasons: For starters, Rosa's already-decent grab game is further bolstered by an opponent who can't crouch under her grabs (like Kirby), and with Luma, all of Rosa's moves consist of more than one hit.
With defaults, Rosa's SideB also cancels Ryu's specials and approaches nicely, and Ryu doesn't get much off of blocking it either.
The combination of Luma and Rosa's DownB prevents Ryu's fireballs from being too effective.
However, there's the difference in weight and killpower: Ryu can afford to play somewhat haphazardly here because any incidental smash attack will hurt Rosa quite hard, while Ryu's above-average weight makes Rosa have to work a little harder than usual to kill him.
When Ryu's offstage, he's in trouble.
I have seen firsthand Rosa's DAir clearnly spiking Ryu out of his SideB, and both his standard UpB and 623 UpB (in the case of the latter, Rosa will need to DAir outside of the reach of Ryu's fist, at the end of the UpB's jump, or whenever the UpB snaps to the edge, in order to beat the more-thorough intangibility)
When caught in the air against Rosa, her Luma-enhanced aerials prevent Ryu's DownB from helping, meaning he has to handle the juggle like everyone else would.
When Rosa's offstage, the flexibility of her UpB's timing and trajectory, combined with Ryu's inexistent air-acceleration can make her hard to intercept.
Ryu will need to take advantage of air FADC's if he wants to mix-up his attempted intercepting-patch.
All in all, with Luma, Rosa in neutral is just too good against Ryu in neutral.
If Ryu tries to walk, move, zone/space, or dash towards her on the ground, Rosa can outpoke with SideB, her jab, or tilts.
If he tries to come in from the air, he's asking to get swatted by Rosa's anti-airs.
If he tries to use fireballs, they will be eaten by either Luma, Star Bits, or Pull.
If he tries to approach with tatsu's, whether on the air or ground, he may take out Luma, but Rosa can simply block or move around it to punish afterwards.
However, with Luma out of the fight, Ryu can get in for free, but I find that he has too much trouble getting past an intelligently defensive Rosa, who still outreaches Ryu on the ground with FTilt, can still swat him out of the air with USmash, beat out careless attempts in with DSmash, can still just as easily grab him, throw, then regrab upon the predicted landing trap, or do other things to stall for Luma's return, while preventing Ryu from gaining too much offense and ground.
Without Luma though, Ryu can safely apply his NeutralB and DownB, potentially get the crumple and dish out heavy damage; Rosa's only multi-hit move alone is her unsafe FAir.
In defaults only, I'd say Rosa's favor, +1
With customs, Rosa gets the obvious Shooting Stars, which are a real pain for Ryu.
Unlike the default Stars, Ryu can now focus them, but it doesn't help that Shooting Stars just beat out his normal fireballs, and have such long range, and also stuff tatsu's.
Luma warp can surprise a Ryu trying to zone or focus, and against a focusing Ryu, the warp can break the focus armor, and Rosa can have Luma immediately follow up to hit the now-armorless Ryu.
With customs, Rosa only stands to gain benefit, while Ryu doesn't which, by proxy, makes him suffer.
With customs I'd say Rosa's favor +1 or +2
