I agree that describing the transfer as a "nightmare" is probably verging on hyperbole, but it is the least straightforward/convenient one to do at LHR.

I suspect the train option is not favoured because those services only run approx every 15 minutes so there's the potential that you might end up waiting around a fair bit (only a problem if timing is particularly important, I guess).

I have used the bus before and it was fine, the only slight issue was that I had to wait a bit longer than I expected for the bus to show up at T5.