My biggest problem with Digimon fusion is that there's a more interesting plot going on in the background while Taiki, Tagiru and friends are messing around.
You know, all the previous Digimon casts teaming up to fight a massive universe ending threat, The only ones Taiki actually meets are the leaders of each of them (Tai, Davis, Takato, Takuya and Marcus)
The main reason I love Digimon Fusion is twofold personally:
A) the animation is clean AF(especially considering the previous anime's overall had... average animation)
B) it takes everything I like about Digimon and ups the ante.
Like, hot take incoming, but I've never really cared for the tamers and their motivations/arcs. I've always been more interested in the Digimon themselves and the insane forms they can take, the lore surrounding them(the Royal Knights for example) and Digimon Xros Wars(I'm honestly not sure which name I like better tbh) has some of the most unique and interesting designs I've seem.
I mean sure the plot isn't anything special or anything, but to me it had a lot of charm when compared to the previous seasons in a similar way to Frontier: it took the basic concept of Digimon and tried something a bit more ambitious.
the 3rd season(the boys who leapt through time, I think it's called?) is where I lost most interest. It lost one of the things I really liked about the show(the really vibrant artsyle) and I just couldn't get over how dull the new one looks half of the time. Let alone the very noticeable dip in animation quality. The plot also kinda fell flat on me because while I think the CONCEPT is at least alright, it went almost entirely the wrong way i wanted it to. The appearance of the previous protags is cool but again it kinda fell flat because it's executed pretty poorly, IMO they should have been introduced on-screen earlier on and did more than just murder a couple Diablomon and Malomyotismon at the very last possible moment.
Overall, for a Digimon anime, it's pretty good. Like... nothing jaw-droppingly amazing, especially if you're interested in the plot and not the lore, but I felt the first 2 seasons had enough charm to make it work for me.