Possible cons? 

) What are the failure scenarios for the pre-cooler?  How gracefully/gracelessly will the engine fail?

) Is there a critical point in the mode switch where the rocket mode (and the various cycles that are meant to sustain propellant transport within the system) might not "bite"? Again, what is the failure scenario?

I would be happy to see an aeronautical engineer comment on this, even though public sources on how this engine would go arse-up are probably non-existent... (don't get me wrong, I wish this engine to succeed.)88.110.112.185