Before I started building my latest layout I experimented by building stock and track in P4 in the form of a shunting plank. I soon realised that converting diesels to P4 was much easier than converting steam locomotives. I also realised that whilst it was possible to get P4 to work, it took much longer than in other scales. This was, of course, entirely due to my own incompetence and lack of skill but I intended to build a reasonably large layout and I seriously worried that I would never get it finished if it was built in P4. I do believe that a large main line layout could be built in P4 but it would be difficult for only one individual to build it. However a team of experienced modellers should be able to succeed.
Therefore I decided to build my layout in EM which seemed to be a reasonable compromise and which has proved only marginally more difficult than OO. The layout is a model of Andover Junction on the line from Waterloo to Salisbury in about 1957 so there have to be lots of Bullied pacifics, King Arthurs etc. However if I was building a contemporary model of Andover Junction or one set in the last, say, thirty years then I would seriously have considered P4 as the line would obviously feature only diesel locomotives or units and the track plan would be much simpler. So really it does depend on your skill level, whether you have any help and also the type and period of railway you wish to model.
Sandra