So I think we're discussing a complex topic. A agree with much of what has been said here and can see where both sides of this discussion are coming from. I've been struggling with getting my thoughts out in a coherent fashion for this post, and I'm currently working as a paid writer!
I'm not an expert in Aikido, but I've done several years of training in the past and was pretty skilled at Hapkido, which is a related art, back in the 90's when I was training in that art regularly. I think there's a lot to unpack.
To start with, I think it's pointless to talk about Aikido in relation to MMA. I think they approach MA from almost opposite perspectives. Aikido is bad for fighting in a competitive sense and it's also bad fighting in a chest beating, bar fight for sport, or bar fight to prove your machismo kind of way. In my opinion, if you need to "win" a fight with Aikido (in the competitive or chest beating kind of scenario) you're going to do significant injury to your opponent, or you're going to lose.
And as a side note, it seems like many people see "chest beating" scenarios as their most likely self defense situations, so I think that's another complication in discussing whether Aikido "works" or not. As someone who was never much into that sort of thing and is even less interested in stupid primate dominance games now that I'm over 50, this scenario is pretty much irrelevant to me, but I think it does lead to a lot of miscommunication when one person says, "Aikido can be good for self defense", and the other person hears, "I can use Aikido to show up (or be) the bully in the bar!".
If you aren't in a position to arrest someone or flee from someone immediately after applying a technique, much of what Aikido offers to end a conflict are things like breaks and hard throws. The problem with breaks is obvious in this regard, and if your adversary doesn't know how to fall (maybe if they do) the problem with hard throws should be obvious too. If they do know how to fall, and you aren't in a position to leave after throwing them, you're kind of left with breaks and throws that can't be safely rolled or break fallen from.
These same issues apply to MMA. Avoiding your opponent, even if you're good enough by some miracle to do it consistently for 5 minutes at a time within the constraints of the ring/octagon, against a skilled assailant, isn't going to win you any titles and is going to bore the spectators to death after about the first 30 seconds. Throwing your opponent is similarly unhelpful. Even if you can consistently pull off those throws against a skilled opponent who knows how you fight, what then? They're going to keep getting up and hitting you or trying to submit you, or you're going to throw them in such a way that they can't get up. Intentional breaks are obviously not OK in a sporting context.
The reality is that Aikido applied properly, at any level of skill that I achieved anyway, is better for responding to being grabbed than for trying to grab someone else. When you chase hands, wrists and arms you're doing it wrong. I feel that it has some cool approaches to avoiding and redirecting an assailant's attacks. Aikido can be applied proactively, rather than reactively, but it's harder and I don't think a lot of people train in ways that are conducive to understanding how that works. Overall, I guess I'm saying that it has a stronger defense than offense.
Having done a variation of Hapkido that had a strong emphasis on striking, I think that Aikidoka would have an easier time setting up proactive throws and locks if they were taught practical striking techniques, but in my experience, most Aikido schools do not do so. I'm not trying to say they should either, just simply stating that it would open more options if they did. I'm not an Aikido historian by any stretch, but these factors do make me wonder about the quote, attributed to Morihei Ueshiba, that Aikido (or combat, depending on your source), is 99% (or 70%, or whatever) atemi. Regardless, it would make the art more applicable to the octagon and for chest beating altercations.
So, to sum up, even if we were to just simply assume that a skilled Aikidoka could pull off the throws and locks that they're taught, against a skilled opponent, it's still not particularly applicable to MMA, nor is it useful for what a lot of people consider to be "self defense"*. I'm not weighing in right now on whether it works or not, simply stating that even if it did, it wouldn't be good, as practiced, in these scenarios. This is an unsatisfying answer for those who want to see everything tested, because it doesn't fit into the usually accepted tests very well. For example, Tomiki Aikido has competitions, but the ruleset is pretty limited and doesn't really try and isn't intended to answer these questions and I've already discussed why the things Aikido is supposed to do aren't very useful for MMA.
If one wanted to test their Aikido in this fashion, something like Kung Fu Wang's standard of being able to avoid 20 of your opponent's attacks in a row to the head, or being able to throw your opponent 7 times in a row, might be more appropriate, but there certainly isn't any existing format for this, and because of its somewhat reactive nature and common philosophical underpinnings, it isn't something that seems likely to evolve out of usual practice. Whether it should or not isn't for me to answer.
*Edit to add, when I say "self defense" in this context, I'm talking about people who think that "showing the other guy they're tougher" or "winning" a bar fight is self defense. I don't think that's valid self defense, but an awful lot of the "self defense" I hear people discussing on the Internet seems to fall into these categories, or not too far off anyway.