From the 737 training manual:
'Asymmetrical reverse thrust may be used with one enhine inop. Use normal reversing techniques and procedures. If it appears that rudder authority is becoming limited during deceleration, return the lever to the reverse idle detent'
If Boeing say you can do it, then it must be ok. I agree that if you were at x-wind limits on the wrong side, then rudder authority could become limiting. I've used it several times, and noticed no swing, plus always use it with one engine inop. in the sim.
Our performance manuals state that with one or both reversers inop, the LDR increases by 6%, and the RTOW by 750kg (dry), 1400kg (wet). Also, for wet rwy performance, engine out reverse thrust is assumed.
You say you wouldn't use assymetric reverse for landing, but you all brief to use it for an RTO every day! If an engine fails below V1, you close the thrust levers, select the speedbrake and deploy max reverse thrust on the operating engine.