I'm middle. In most ways I don't mind. My dm was also middle so she was aware and made sure that I didn't too much get hit by "You two older ones are too old to go to the Christmas party but you two younger ones really must get to bed now." I was almost exactly in the middle with 3 years between me and each sibling.
But, in common with every family, there were issues.
One of mine was that #3 was difficult, #1 not easy, and I am fairly certain both #1 and #3 would have been diagnosed with ASD now.
I was always the one expected not to make a fuss and compromise to make life easy. I normally did. If I did try making a fuss I was treated as disappointing (despite making less fuss than the other two did) my parents. One example was when I was about 8yo, #3 was nearly as tall as me and was making a fuss about their bike being too small. So mine was changed for them. No we couldn't share-they needed stabilisers. When I complained I got the comment that their bike was too small so I was being mean because if they hadn't done that they wouldn't have had a bike. It was 2 years before #1 grew out of their bike and it was able to be passed down to me.
However I think that was personality, and at times I've gained hugely, especially with df for whom compromising normally means he gives back far more than you expect. The other two argued, so he stayed firm. They said that wasn't fair. They still do. I say I just learnt how to handle him. My gain.
On similar subject, they'd say I was always jammy. Actually we say it about my middle one too. But I'd argue that actually a lot of the time they said I was jammy it was simply that I handled other people a lot better than they did.
Other issues that you could say were being the middle, were probably more to do with having girl, girl, boy. So we were expected to help far more in the house and drop everything to do it, which I suspect was a lot to do with being girls than being older, and also #3 made so much fuss if asked, that it wasn't worth the hassle.
It was a standard thing if all three of us were set to do a household task, #3 would pick a fight within 2 minutes and walk off in a huff, and we'd be told off for "upsetting them" and they wouldn't do any more.
I used to feel I only got 2nd hand stuff. Because #1 would pass down to me, then #3 would get new because he needed the "boys'" one. I remember one time being got a new coat, I think almost my only brand new one until teenage, and it having to be a blue boys' one so it would pass down to #3, while #1 and #3 got to choose. As a parent, I can sympathise - we didn't have much money, the surprise is more we got new coats. Maybe my grandparents paid?
I think the thing that did effect me as the middle was presents. On about three or four occasions #1 got a present I really envied. And I asked for the same, for my next birthday/Christmas. On each of those I was told I wasn't allowed it because it wasn't fair on #1 if I got them 3 years younger.
On each of those #3 got the item at the same time or for next birthday/Christmas because "it wasn't fair if they were the only one who didn't have them." And no, it wasn't items that meant they couldn't join in without, eg one was a watch.
Similarly a lot of rules that were set for me and #1 when #3 was too young for them, never got set for #3. Bedtimes were one example. Mine was always 30 minutes before #1. #3 didn't get given a set bedtime, so it ended up being they went to bed at the same time as me.
Most of things I felt were unfair as a child I can see, as a parent, had their reasons behind and can accept that even if I didn't like it, it wasn't done for bad reasons. The presents, I can't see any good reason at all still, other than stopping #3 stropping about it, which actually I don't think he was that bothered by the items, so I don't think he would have.
But I had 3 myself so I can't have minded too much about it, and although my middle one would tell me she's the most hard done by and both the others have it far easier; the other two would disagree thoroughly and they are, for the most part right. 🤣
Edited to say I've thought of something that did effect me:
#1 was a very hard worker and clever. #3 was meant to be so clever that adults held their breath when they talked about him. And I was piggy in the middle doing okay, but not expected to do brilliantly.
So I stopped working except the necessary. It was easier to not do the work, then tell myself that maybe if I'd worked as hard as the other two that I'd have done as well than accept the evidence that I'd worked hard and not done as well.
When I was an adult I was thinking about this, and realised that actually I did better at GCSE level, and at A-levels did the same. It honestly never occurred to me as a child, and it was never said to me either. So either the other two didn't work as hard as I'd always been told, or they weren't as massively cleverer than me as I'd always been led to believe.
Sometimes I wonder if I'd worked hard how well I'd have done.
I suspect that was again more to do with personalities than being middle though.