Some radials were more "robust" than others and some liquid cooled engines were more "robust" than others.
I don't think anybody is talking about "relative invincibility"but rather the ability to keep running at 30-50% or so of full power in order to get home, not keep fighting at 90-95% power.
The R-2800 and some others had three main bearings so a missing cylinder or two would certainly throw the engine out of balance both in firing order ( although not much worse than a couple of bad plugs) and physically due to flopping pistons but in reality the pistons may have gone with the cylinder or been battered to pieces in short order banging against the crankcase. It rather depends on how far down in the cylinder the skirt went at the bottom of the stroke.
Some radials ( most/all service French engines, the Russian M-88 and a few Italian ones) had two main beings for a two throw crankshaft and were operating at the limits of crankshaft flex to begin with. Throwing them out of balance might have resulted in excessive crankshaft flex and failure (broken crank) in short order.
Other variables are the type of bearings, ball, roller, plain and of the plain bearings you had a variety of materials. Ball/Roller bearings will actually run for quite a while on little or no oil pressure.
Differences in radial engine damage can be entire cylinder barrel gone and piston (or remains) flopping about, cylinder head gone but cylinder remaining so piston is riding in the bore ( keeps engine in balance), damage to head causing valves either to stay closed (rockers or push rods shot away) or valve retainers damaged allowing valves to fall inward and hit pistons. Usual result is bent valves and dings in piston top. It is possible to hole the piston. But you can do that by severe knocking or pre-ignition. Worst case in a holed piston is (in addition to loss of power) is pressurizing the crank case and blowing oil (or oil vapor) out the breathers. Small particles in the oil aren't good long term. A large amount of debris can block oil pick up/return tubes.
Engines of both types can continue to run (obviously not at full power) with miss-firing cylinders or cylinders with damaged valve trains. I once had a timing chain jump on a V-8 engine, 16 bent push rods and 8 bent intake valves, exhaust valves were smaller and if bent slightly could be reground. the engine "ran" but would not make enough power to move the car on any kind of slope.
Plenty of engines of both types have continued to run with broken valves, valve springs, rocker arms, or followers.
On the other hand a hit to the carburetor could spell the end of some engines no matter what type they were. Hispanos and their Russian off-spring being a bit better in this regard with 6 carburetors, one for every two cylinders, but then you are much more likely to hit a Hispano carburetor.
http://www.enginehistory.org/Museums/SNECMA/157 H-S 12Y.jpg
V-12s have larger areas of vulnerability to small arms ( non-cannon)fire . How each particular engine responds to such damage may very well be different but there seems to be little doubt that the radials, in general, were less vulnerable ( but not invulnerable) to combat damage. I
repeat IN GENERAL as one or two radials were rather notorious for not taking combat damage well.