The Gamecube Star Fox games had their flaws but were good games, just not good Star Fox games.
Star Fox Adventures was a Zelda-like game with simpler game-play and an easier difficulty with a few Arwing levels thrown in.
The graphics were great and the Dinosaur Planet setting was a nice change but you could easily breeze through 100% of the game.
Overall, SF Adventures had very few classic Star Fox Arwing levels and lacked the depth and difficulty of the previous Star Fox games.
Star Fox Assault was more of a return to classic Star Fox game-play with Arwing (spacecraft) & Landmaster (tank) levels.
They also had the new addition of "on foot levels", which were liked by some and disliked by many others.
Another change to classic Star Fox formula was making the game linear, with only one path from start to end.
Below: Map of Star Fox 64 which is non-linear with lots of paths to choose from (Star Fox Assault had ~10 levels total):
Yet this made SF: Assault longer in a single play through than Star Fox 64 and more difficult than SF64 depending on the path taken.
Overall, SF: Assault's linear path made the game have less variety and depth than Star Fox 64, and foot missions a divisive addition.
Below are some Assault concept art drawings via siliconera:
