I've yet to feel legitimately challenged in Spirit Mode. That's not to say I haven't had a hard time, or that it's been smooth sailing as I've traversed the board, but I haven't run into anything that's tested my skills or forced me to adapt how I play.
For anyone who doesn't know, Spirit Mode is Smash Bros. Ultimate's version of a single-player campaign. You start out as Kirby, and you make your way all over a giant map and complete challenges as you go. Some of these challenges unlock a new fighter on the roster to play as, and all of these challenges are some variation of a basic Smash match. You're pitted against a CPU or a team of CPUs and are tasked with taking them down. The twist in this mode is that you can equip Spirits, which are pictures of various characters from other franchises that act as stat buffs and come with abilities or traits, such as increased damage with Magic attacks or the privilege of starting a fight with a certain item. These Spirits can be leveled up to increase their effectiveness in battle, transformed to gain new traits, or trained to infer additional buffs and debuffs.
However, the enemies that you're facing also have these Spirits equipped and the fight is lightly themed on the spirit. For example, a fight against Mario with a Hal Emmerich Spirit turns this to fighting Dr. Mario alongside a giant metal R.O.B. Get it? it's like fighting Dr. Emmerich and a Metal Gear. These are generally fun little nods to characters which couldn't be included in the game, and each and every Spirit battle has a different theme, even if you do start to see repeat effects after a while. There are only so many variations of fighting with periodic screenshake against the Spirit of a large enemy, after all. Once the enemy is defeated, you gain the Spirit for use in future battles.
The issue with this mode is that the idea of difficulty isn't present in a satisfying way. Many of these fights are over in a few seconds if your Spirit is strong enough and you choose the right type from the effectiveness triangle (Shield, Grab, or Attack). Once the battle is done, you pick up a spirit that likely does the same thing as a handful of other spirits you have, and then you move on to the next dot on the map to repeat. Occasionally, though, there is a particularly troublesome battle in front of certain pathways or treasure chests, just to make sure you can't stomp your way around the board with no resistance. These battles almost always have the same conditions: Fight 4 of the same buffed enemy on a stage where there is a negative event that is especially difficult for you.
Cut to you being bounced around the stage helplessly, trying out different item cards between rematches, until you luck out on CPU behaviour or get a lucky hit in. I've "won" some of these battles because CPUs have just run off screen because of an item I was holding, or because they chased a healing item off of a transforming stage.
Fighting 4 Yoshi, fighting 4 Inkling, fighting 4 Kirby, fighting 4 Wii Fit Trainer, etc. with no team damage for CPU and powerful Assist Trophies or stage events (such as earthquakes causing tripping, wind blowing the player away, sticky/lava/electric floors that don't affect the enemy) is not a very fun time. On top of that, having to use fighters that are unlocked sequentially makes progress on these stages difficult. The weaker battles are too easy to force familiarity with any one unlock, but the harder battles are too hectic and frustrating to learn effective strategies from. Sometimes, you have to use up Spirit slots that would have been for items or increased damage just to negate the stage events.
There is a lot of Single Player content in Smash Ultimate, but it's nowhere near the level of Subspace Emissary, which was an actual journey through a unified story mode, complete with cutscenes and character moments. Spirit Mode is a glorified Special Smash that rocks from being a breezy, fun time to becoming teeth-gnashingly unpleasant, forcing you to grind out Spirit cards and use characters you don't enjoy playing, and giving up abilities that might give you an edge in a battle just to get back to neutral. I find myself pushing through it just because it's a reliable way to unlock characters, rather than out of an actual desire to see the end of the mode.