I've has some time to collect myself and I think I can enunciate why CotM2 has gone off rails compared to the first game, which was excellent.
It's got a case of "sequel 'cleverness' syndrome". The first game is just a clone of CV3, arguably an improved clone. It doesn't really stray far from the original and it doesn't mix up the overall gameplay too much. Now the sequel on the other hand decided that it has to "mix things up" and, as often the case, it didn't turn out as well as planned.
Three issues come to mind specifically.
>1. Stage Hazards
While the first game has wind on that one ship stage and had you running from a horde of hellbats on the final stage, it was otherwise a conventional classicvania experience.
In comparison the sequel goes absolutely balls deep on stage hazards from stage 5 onward, culminating on an unkillable ball of destruction that dogs you through the last stage. And unlike running from the hellbats in the final stage of the first game which just functioned like an autoscroller that pushes you to progress quickly, the ball of hellbats in the second game requires you to constantly manage it by knocking it back which not only slows you down it also divides your attention away from platforming and dealing with the monsters in front of you.
>2. The Party
In CotM your party was just a set of alternate (and optional) characters. You can easily complete the game with just Zandatsu or with Grandpa Sypha, Girl Trevor, or Not-Alucard or with any combination there of.
The second game instead is tuned around you having all 4 (not optional) characters and if you lose one of them you're basically fucked. Gun Guy in particular has a really small health pool and a slow attack speed so it's easy to lose him.
Specifically the abilities of Spear Bitch and Dog. Spear Bitch lets you transform MP into a large health drop for anyone to pick up. Dog just has an invincibility button at the cost of something like 1 MP per second. Obviously both of these abilities could be very abuseable and could trivialize large parts of the game. So in response they ramped up the difficulty to solve the situation that they caused by giving them those abilities in the first place. There are several screens across multiple stages from stage 5 onward that basically require you to run through with the dog with invincibility on to not take too much damage from attrition.