Speaking as a current DC-6 Flight Engineer, I would say most of the info here is true. I just have 2 small corrections.
Someone mentioned that radials would be easier to start with magneto impulse couplings. There are a variety of ignition systems used on the round engines, some work better than others. Eg, the R-2800s on the DC-6 will frequently start on the mag alone before I even touch the ignition boost. (That's why you leave the mags off while checking for hydraulic lock on the starter.) By contrast, our C-46s also run R-2800s, but with slightly different ignition systems, and they will not start without boost. In any case, the boost works just fine.
Second, the statement was made that the big radials have poor induction distribution compared to horizontally opposed engines. I believe it's the other way around. The proof is that the radials are regularly run lean of peak from the factory, even though they are carburated. A Lycoming or Continental carburated engine will run very poorly lean of peak. Even the injected flat engines require after-market balanced injectors to run lean.
Re # of blades, on an R-2800 with a 3 bladed prop, it takes 6 blades to complete 2 full revolutions of the engine and check each cylinder for hydraulic lock. Anything after that is for pre-oiling, particularly of the prop reduction gears. We use 15 blades for a cold start, 9 for a hot start within 1 hour at cold ambient temps, and 6 for a hot start within 1 hour in the summer. We never pull through the prop by hand, just use the starter. Don't seem to have many problems. The C-46s have a starter clutch to protect the engine, I don't think the -6s do.
As for whether or not a radial is hard to start, I guess it would depend on what you're comparing it to. Obviously, on a modern car with numerous sensors, and computers tweaking the fuel, air and ignition timing several times a second, you'll have more problems maintaining the electronics than you will getting it started.
On the old radials, there are numerous actions which must be taken by multiple people; when those actions are done correctly, the engine usually starts easily and reliably. When there are problems, you just have to guess whether it's too lean or too rich; that's where experience comes in handy.