I think one of Fedden’s concerns was probably with how crowded it would be under
the cowling leading to problems with proper airflow for cooling. The Alfa Romeo 135,
basically a twin-row Mercury, was plagued with overheating problems.
The intake air feed to the front row also looks problematic, and the close proximity
between the exhaust ports of the rear row and the intake ports of the fwd. row are
probably something that didn’t help with performance. The aft exhaust piping would
be heating the fwd. air intake trunks, internal combustion engines perform best with
cool intake air, and as the 135 didn’t have an intercooler the intake air would already
be carrying the heat added by supercharger and would be heating up even more as
it passed through the heat zone of the exhausts, not a good combo for performance.
The close cowling and poor airflow would also be increasing the temperature of the
intake trunks.
The 135 was in development from 1934 until 1945 without becoming a production
engine, which suggests to me that Fedden’s doubts were valid.