The issue with phasing is it actually still requires the Speedster to unphase themselves inside the person. As Cisco described, when he remembered RF killing him via chest phase, Thawne phased his hand through Cisco's chest and physically crushed his heart. Now, that is all good and well with individuals less durable than he is. But Kryptonians aren't exactly standard. If he tries to solidify his hand in a place occupied by tissue far tougher than his own, he could very well mangle his own hand instead. Not saying it will happen. But I wouldn't totally discount it either.