If the Solíno Military were composed of ex-Black Hand units plus those others, than they would definitely have an advantage.
Even if the Army of Chaos and new recruits apparently cannot go up against the world's most powerful private army (which is total BS), they still have ex-Black Hand operatives who would have months if not years worth of experience against their former operatives.
And even if by some chance the ex-Black Hand Solíno Military cannot defeat their former operatives by themselves, strength in numbers beats anything.
Also, depending upon how far one is in the storyline of Just Cause 4, the Black Hand units fighting the demons in Los Demonios can either still be Espinosa-loyal Black Hand or defected Black Hand units to the Army of Chaos who haven't changed their uniforms/camouflage scheme yet. If the latter, they have been fighting the demons for at least days. If they can handle a centuries-old plague, they can handle pretty much anything.