I don't have to motivation to finish the chain grab frame data at the moment (and likely won't for a long time, if ever); it's tedious to collect and the vast majority of it is practically useless anyway. If I could make it significant more efficient to collect by automating the process then I'd probably resume doing it, but until that's possible I don't see myself working on it anymore.
I'll write about the chaingrab in practical terms though; it's not really complex in practice. To my understanding:
Up throw chaingrab to 35% into up throw->dash->jump cancel->up smash (no direct combo off the up smash if they DI it behind), or down throw->down smash (down smash doesn't connect if they DI the throw behind), is close to optimal when not taking platforms into account. You could chaingrab for a little longer but then it gets more difficult that I'd like for the reward.
You could also down smash non-behind DI instead of dash->jump cancel->up smashing all DI. However, I don't believe this is practical to do consistently (if at all); because of the small window for covering DI away on the up throw with an up smash, I don't think you have time to confirm the DI before dashing. Therefore, you have to start dashing immediately, and then if they didn't DI away, do a pivot up smash or early up smash.
If you could react to the DI fast enough that you didn't have to preemptively dash, then you could down smash non-behind DI for a guaranteed followup. Since it's a matter of reaction time, some players may be able to do it with practice, but I don't feel like it's possible (at least for me). Reaction time is difficult to take into account objectively, because it varies so much with the individual and the situation, and because the amount of data on the subject is lacking.
Even though you don't get a direct followup from the up smash if they DI it behind, you still get a tech chase. Also, if they don't DI the throw behind, then depending on the timing/positioning of your dash/jump cancel/up smash, the up smash can be reversed; which makes it a 50-50 guess as to which direction to DI the up smash to avoid the followups.
Optimal DI on the throw at 35% is always behind regardless of whether it's up throw or down throw; the opponent could try mixing up DI on the throw to throw off the Luigi, but the up throw chaingrab is guaranteed regardless of DI, and DI behind has a significantly smaller window to catch as well as not being subject to the DI mixup on the up smash. Because of that, and because Luigi doesn't have any useful DI traps off grab at this damage, I believe that up throw to up smash is the optimal way to end the chaingrab. While down throw down smash does work, any opponent who knows the matchup will just DI the throw behind; at least with up throw up smash you get the up smash before their DI behind gets them out, and it covers all DIs on the throw. In a throw out of neutral (where the opponent might miss the DI), or when platforms are in play, then down throw down smash could be better.
The chaingrab is not guaranteed 0 to death, but it's good and guaranteed at least 0 to 45 or so, with more generally guaranteed/favourable damage into edgeguard/kill setups afterwards. The only hard part of the regrabbing itself is reacting quickly enough to cover both DI in front and no DI/DI behind at 0%. No Luigis I've seen go for the chaingrab properly for some reason (even on FD) and usually go straight for up smash, so either they know something that I don't or they just don't know about/want to use it.
I geuss just the extremes; highly favorable and highly unfavorable MUs. Anything close to even - 40 to 60 can usually be subjective or bias.
From what I've seen it seems that Luigi has a lot of difficulty against against Marth, Sheik, Ganon, and Samus. Spacies seem to obviously be an unfavorable MU as well, but I'm unsure to what degree.
Ganon and Samus can be hard until you learn them, at which point I feel that Luigi wins. There was a thread on here where that was discussed briefly:
http://smashboards.com/threads/luigis-worst-matchups.385815/
Spacies and Sheik are borderline unwinnable (possibly actually unwinnable), though spacies are far easier than Sheik below high levels of play or when the opponent doesn't know the matchup.