Water had a buff because had become with 4.0 simply the worst main available. All the new ninjas added simply worked better with other mains, even water attribute ones like edo tobirama and shark kisame, ignition was too common, making so her healer role less useful and the easiness to get evading ninjas denied all the good sides of her damage dealing role, so she, indeed, needed a big boost and she got it with the double evasion in the place of the useless reserve seal (that any ignition was able to negate fully) and with the chakra stealing of shark bomb (that made actually useful the mystery also against evading ninjas, differently from before).

About fire main the only reason why the immobile mystery was boosted to overcome immunity was simply because before 5.0 for fire main existed 1 mystery only for higher tiers fights while all the other mains had two options commonly used (sand dust/dance of impetus, regenerative healing jutsu/shark bomb, lightning armor/acupuncture seal, boulder jutsu/chakra wall), so they needed to make another mystery appealing enough to induce a player in selecting it in the place of 4th mystery.

In matsuri the stats are spreaded, you basically do not crit against a comparable power or higher power enemy even if you own an overstacked move 1 outside it, and if you face an expendable team with two permanently immune ninjas and a chakra generator that does it even if chaosed (like kurama naruto + kushina habanero + kabuto snake cloak/kisame samehada) you get simply owned even with 10k more power (because in matsuri could be you face teams you never face outside it) . So if you run the shark bomb team with neurotoxin you risk too much (unless you are in a situation like shyrke)

That's because you do not use cc wisely. Ninjas with 0 chakra mysteries (like masked man or normal kushina) still counter her fully even without initiative if she runs neurotoxin.

The problem is that you are compelled to use main talents you do not normally use or to be smart in using the mystries.