Normally, we think of civil cases as being easier to win than their criminal counterparts. The standard of proof is lower ("preponderance of the evidence" versus "beyond a reasonable doubt"), and many activities which are not subject to criminal penalties might nonetheless carry civil liability. There's a reason why O.J. Simpson was acquitted of murder but nonetheless lost the civil suit against him for wrongful death.
But, at least in the context of police brutality cases, there is one hurdle present in civil litigation that is not found in criminal law: qualified immunity.
Qualified immunity is a judicially-made doctrine that shields officers of the state (not just police officers, though they're the most common subjects of litigation) from civil liability for constitutional violations unless they violate "clearly established" law. In other words, it's not enough for the police officer to have violated the law, it has to have been obvious in advance that they violated the law. The judiciary has interpreted this in an exceptionally stingy fashion, insisting on extremely granular inquiries into whether the precise fact pattern alleged by the plaintiff had been specifically demarcated as unlawful in a prior case. The question isn't something like "has it been 'clearly established' that a police officer can't physical strike an non-resisting suspect?", it's instead more like "has it been 'clearly established' that a police officer can't specifically tackle a non-violent, non-resisting, non-threatening suspect who weighed 130 lbs?" If one doesn't find a case that mirrors those facts, the law isn't "clearly established" and the case fails. The Supreme Court itself has accordingly characterized qualified immunity as a shield for all except "the plainly incompetent or those who knowingly violate the law." And the Eighth Circuit (which includes Minnesota) -- well, it's insulated some pretty wretched behavior under qualified immunity's guise (and some of its judges think it hasn't gone far enough!).
By its nature, qualified immunity means that many actions which are concededly unlawful violations of Americans' civil rights are nonetheless protected from civil suit. But there is no qualified immunity in the criminal law: one cannot escape criminal punishment by arguing that there has not been prior case law "clearly establishing" that the conduct you're accused of is unlawful. I'm dubious about the ultimate viability of criminal law to serve as a systemic brake on police brutality -- I'm not sure that is a task it is well-suited for (though it is certainly appropriate in particular cases -- the George Floyd case appearing to be an obvious). But a criminal prosecution -- as much as it is (properly!) hamstrung by heightened burdens of proof compared to a civil suit -- does evade the strictures of qualified immunity. And given how aggressively the judiciary has interpreted qualified immunity to shield bad actors in the American policing system, that's a virtue which cannot be discounted.