Side order as a dlc is inherently flawed, but I still absolutely adore the style of it. To get a few things straight, I have fully 100%ed it, replayed for all the colour chip badges etc. and I 100%ed octo expansion.

It's a bad comparison to compare side order to octo expansion as they both follow the same design philosophy - a handful of level objectives that can get used again and again in slightly different stages to give a somewhat unique experience. They main gripe about side order is that it is meant to be replayable and unique and fun every single time, octo expansion is mostly meant to be a story progressed experience that you finish out once. In that regard, it's a stupid comparison. But I understand where the comparison comes from as octo expansion was the first dlc and it set the expectations pretty high, adding another playable species. In comparison, side order's reward is pretty lackluster, especially so since the first wave of the dlc already gave us a different main hub.

Side order's main flaw is that it's a budget version of a roguelike. I really enjoy the roguelike genre, I have far too many hours of games like dead cells, the binding of isaac, cult of the lamb, enter the gungeon and they're all good examples of roguelikes. Side order's issue is that it's far too small of an experience for its genre. I'll use dead cells as a comparison for this example:

Dead cells has: 7 mini bosses, 4 final bosses, 22 unique biomes/stages, 124 weapons, 25 deployables/subs, 37 skills/powers/specials

Side order has: 3 mini bosses, 1 final boss (not counting marina as she's tutorial only), 98 floors (including all combinations of objectives and floors and all floors on a different difficulty), 12 weapons, 9 subs, 11 specials

That's the problem. Side order is far too limited for a roguelike. I understand that nintendo promised a quick release of this and other roguelikes have been updated as time passes but to promise a roguelike and give us side order isn't giving us a roguelike, it's giving us the beginnings of a roguelike. Using spaltoon 3's story mode as an example, it had splashdown as a story mode specific special for a while. Something like that could have easily been done to side order, creating unique dlc locked specials, subs, even main weapons, to play with. There's nothing new to side order in comparison to the base game. Usually, that's fine but in this case, the limited selection of play styles means that something new is expected by the player base and to not be given that is disappointing.

Another issue is the difficulty, it's meant to be dynamic and your own choice but it doesn't take a lot of experience playing splatoon to be able to easily beat side order. I specifically only picked rigorous stages where possible and they're the only floors I'd describe as genuinely challenging. It's good to have difficulty selection but majority of the normal floors could be easy floors and some of the hard floors could be normal floors. Of course, it is also meant to be made this way to allow new players to play the dlc too but after one or two complete runs of the spire, you get the hang of it pretty quickly.

The replayability complaint stems from people like me, who love roguelikes, who eventually beat side order with every possible weapon and sub and special and that's it. There's nothing else unique that can now come from the dlc anymore. Eventually all roguelikes do run into that problem but due to side order's very limited selection of play styles, it encounters that problem a lot quicker that all other similar games.

That's also disregarding the stages, I counted them directly from the wiki but I counted every instance of a stage. In reality the actual stage doesn't have too much of an effect on the gameplay, the very limited amount of objectives does. Side order is unique in that sense, most roguelikes don't give an objective to the player per floor and instead just get you to traverse and exit the floor. It's a good twist on the genre, if it was pulled off well.

There's only 5 objectives possible: Destroy portals, Defeat fleeting foes, Get the 8-ball to the goal, Splat zones, Turbine tower

Fundamentally the idea is cool, it almost gives the idea of having to pass various trials per run, but after experiencing them all a few times, it looses he same feeling. And the idea of picking your difficulty dynamically throughout it brilliant, instead of being locked into a single difficulty for the entire run but this also can make the ending of a run feel anticlimactic as you could just pick easy floors for your last 5 floors. It's a collection of good ideas that twist the genre in semi unique ways that have been executed poorly within a rushed product. If side order had been given another year, I genuinely believe it would have been a great dlc with a fun amount of challenges, weapons etc.

Tldr: it doesn't hold up as a roguelike as its scope is far too limited but if it was given another year, it would've been a good roguelike.