Nimble escape could give regular advantage for a small number of classes in certain conditions, it’s by no means a guarantee. Even if it was (and would be too good as such) the limited scope inherently makes it less disruptive than the fallen Aasimar. After that you just list the variant human over and over, which has been a source of contention from day one. Do you have any examples that don’t specifically rely on having access to optional feat content?
Now that I know more about Volo's goblins, it seems clear that Fury of the Small is going to do damage competitive with Aasimar bonus damage too. If your 10th level Necrotic Shroud is doing 50 - (27 or whatever) damage over five rounds, and my goblin's Fury of the Small is doing 10 points of bonus damage three times per day, that's comparable, nicht so?
Even then, many of the examples you posted don’t stand up to any scrutiny. You’re going to have to explain how the rogue/sentinel interaction works, because by my reading the only way it’s useful is if the rogue has the reaction attack triggered while also having advantage.
If the reaction attack is triggered, you have an adjacent ally almost by definition. You don't need advantage to get sneak attack damage in that case.
Also, Swashbucklers rock.
100 HP is not better than 50 damage, at least not in all cases. Inspiring leader takes time to set up (10 minutes each) and its value is spread over the party. If the wizard is getting focused down, the bonus HP on the cleric aren’t helping. This leads me to the real crux though, is that killing things faster always saves you more HP than by reactively healing it. Doubly so against high-threat monsters, or those with powerful save effects. Lastly, you can stack multiple aasimar damage buffs, and I don’t believe this is the case with Inspiring leader.
Inspiring Leader isn't reactive. It's proactive.
"Killing things faster" is a way of attempting to proactively heal yourself, but it is a rare combat indeed where doing 50 HP of damage will prevent more than 100 HP of damage to the party. It only works that way against glass cannons, such as a wizard who is about to cast Cone of Cold on the whole party, or a young white dragon who is low on HP.
Mounted combat is barely worth addressing. Mounts can’t go into a wide number of areas, it only affects medium creatures (which many of the more dangerous foes are not), the larger weapon die works out to be +1-2 at best, and you also have to figure out how to keep your mount from getting hosed by AoE effects, which was a big concern even with the old Ranger’s animal companion. I think your post is the first time I’ve seen the mounted combat feat brought up in positive fashion.
In the end, all of these options are also available for the aasimar character, at the price of missing out on a stat bump or a different feat.
I'm not going to get into a debate with you about what kinds of terrain are most common at your table. If your campaign takes place entirely inside and/or underground, then two things are true: (1) You shouldn't take Mounted Combatant, (2) Sunlight Sensitivity is not an issue at your table, so kobolds and drow become more attractive.
Your comments about Large foes puzzle me though. If you're fighting Large foes, how can you be in a place where a Large steed is not allowed? In such a scenario you aren't gaining advantage bonus, but you're still gaining the offensive bonus of +2 damage to every attack (by using a lance) and the defensive bonus of higher movement and free dashes/disengage from your mount. That's a clear win. Note also that the aforementioned higher movement/free dashes/disengages also helps to protect you and your mount from AoEs, by staying out of the dense clumps of PCs which make AoEs attractive.
Your comments on AoEs are suspect. On the one hand, I'm not going to tell you how common AoEs are at your table. On the other hand, much like your comments about mounts not being allowed to go "many" places despite fighting Large and dangerous foes, it smells like an objection based in theorycraft instead of actual play. Sure, if a mount bites the dust to an AoE attack, you'll be inconvenienced for a while until you can get a new one. You may have to draw your longsword instead of your lance, until you can cast Find Steed or Phantom Steed or borrow your buddy's horse or get to one of your backup mounts. So what? How often do you actually lose a horse that way? It would have to happen a
lot before Mounted Combatant became worse than Necrotic Shroud.
If, arguendo, Necrotic Shroud is adding 27ish points of damage per day at 10th level, and Mounted Combatant is adding merely 2 points of damage per attack because all of your foes happen to be Large, and we ignore all the defensive benefits of a mount, then if you land one and a half hits per round of combat for six three-round combats, Mounted Combatant is adding 54 points of damage, which means you have to kill a horse every day for Mounted Combatant not to be as good as Necrotic Shroud. (Since that would mean you spend 50% of your time on average unmounted, and 50% of 54 is 27.) Does that much unavoidable non-Dex-based AoE
actually happen at your table? Is every single combat against white dragons?!?
First off, I put the 50% uptime into my original argument, which is incredibly low, since that means both your attacks would have to miss for ½ the combat rounds, since the ability only requires 1 hit per round to activate. If you assume that most/all attacks hit, it becomes even better than I posted. If you assume it has an 80-100% uptime, then now you’re trading 27 damage for 80-100, along with a CC opener, which cuts the opportunity cost somewhat.
So now you're assuming 8-10 round battles, but survivability (such as what you get from Nimble Escape, Mounted Combatant, Mobile, etc.) has no value? Color me skeptical. In such battles, "kill enemies faster" is clearly not working.
The more I read about Volo's, the more Aasimar look worse than goblins.
