I feel that it lacks the fun of Phineas and Ferb.
In Phineas and Ferb the title characters always have some wacky scheme and their part of the show is built around this, while Doofenschmirtz has some idiotic evil scheme that he and Perry's part is built around. By contrast in Milo's law Milo does something and it randomly goes wrong, while there are 2 agents who are doing something and some random person is trying to stop them.
So Phineas and Ferb are proactive as they're doing something to create the plot. While Candice is usually more reactive to what her brothers do she actively tries to show her mother what they are doing. By contrast Milo is purely reactive as he's trying to adjust to whatever random thing happens, instead of driving the plot. So the plots of Phineas and Ferb are more interesting as the characters have more agency.
With Doofenschmirtz and Perry it's always clear in each episode what is happening and why, with Doofenschmirtz usually failing because of his own ineptitude. Regarding the agents I have no idea what they're trying to do and it often fails for random reasons. Again since Phineas and Ferb has clearer goals and the characters have more motivation for what they're doing.
In conclusion Phineas and Ferb is more interesting than Milo Murphy's law because the characters drive the plot instead of being pushed around by the plot.