“Ferrari had a 12 cyl boxer in the 1970’s (Ferrari 512 BB) and Alfa Romeo had a flat 4 in the same period in their Alfasud series.”
Ferrari’s 12-cylinder boxer is, strictly speaking, not a boxer; the crankshaft in their engine looks basically like one from an inline-6 or V12 engine, and Ferrari themselves referred to it as a “180-degree V12”. Using a traditional boxer crank (with the rod journals set 180 degrees out from each other) would have made the engine a LOT longer, and with six cylinders on each bank the engine is pretty smooth anyway.
This engine, shrunk to a displacement of 3.0L, also formed the basis of Ferrari’s F1 engines for much of the 1970s.
“...Ferrari’s 12-cylinder boxer is, strictly speaking, not a boxer...”
Yes, you are indeed right, although this did not keep Ferrari from referring to it as a boxer:
512 BB (Berlinetta Boxer).
VW’s and Porsche’s 4 and 6 cyl engines are also flat coniguration “boxer” engines.