Assault varies because of the way gameplay is so different between levels
There are three scrolling levels in the game, the first level, the 5th level, and the 10th level. All of them play like Star Fox 64, although they are a bit more restrictive in how far you can fly up and down and side to side, and except for the last level, enemy placement isn't challenging to fight against.
The on foot levels have always been the most critical part of the game. While it's okay as a TPS, it plays very primitively and doesn't flow well. Combine this with mediocre weapon selection and repetitive objectives (every on foot level, sans Fichina, is "destroy X number of hatchers/teleporters while also not letting the skies get overrun by enemies") and terrible level design and the on foot levels are pretty bad, with their only saving grace being how multiplayer handles on foot action.
Another problem with Assault is that during the all range mode missions where you get to pilot an Arwing it's pretty lack luster. Enemy AI is nearly non-existent, your allies, unlike in 64, are scripted and don't have their own AI (I watched Falco fly in the same circle occasionally shooting at nothing in one level) and they only act up at specific scripted events where they have enemies chasing them, but they never need any help otherwise, unlike in 64 where they freely engage and are engaged by enemies.
Wing riding was a welcome surprise and a fun part of the only two levels it make it in. Namco shows its arcade shooter roots visibly in these portions of levels, however due to the only usable and valid weapon being the plasma machinegun these levels are basically hold down A and aim, with the real point of the levels being to keep your score combo going.
And speaking of the score combo meter it's something that should have stayed in Starfox games, as it was a neat way to get additional points and to keep the pressure on the player. Zero especially would benefit more from it.
But yeah, Assault is okay, it's not awful but it left me with wanting more (which probably was the case because there are plenty of rumors of the game having development scrapped and rebooted mid-way through and there are hints of one extra level being scrapped in the Titania multiplayer map)
64 3D is a pretty faithful remake of 64. You won't hate it.