The reliability. Last I heard you get about 100 rounds out of the .50 cal before it jams. The 40mm hurt two guys down in Kuwait when they tried to clear it.
Can't shoot while moving. Moving, shooting and communicating are pretty important tasks. Not being able to do all three at the same time is like not being able to walk and chew gum.
What aspect is there not to like?
Not big enough for some?
Go off a paved road or street in one- admittedly not a likely problem in cities- especially if one side's wheels are on pavement while the other side gets in sand or mud, and they roll right over, especially if they're travelling at any serious speed involved. They steer with four wheels in front, not just two, and the high center of gravity combined with four wheels lower on one side than the others, especially once the wheels are cut suddenly to try and get back on the pavement, and over she goes, sometimes just on its side, sometimes on its back, as with the two Strykers that went into a canal while along a top bank, drowning the crewmembers inside, whose rear door exit was chained shut. Rescuers from other vehicles couldn't get to them, and upside-down, gravity holds the two-ton rear crew exit ramp shut instead of opening it.
Thank God we're not up agains other hostile vehicles of around the same size, with enemy weapons systems that actually work deployed against the Stryker. We'd be losing ten guys with every vehicle that burns.