Some special zombie can completely screw you over in singleplayer mode.
Ticker, digger, elemental is mostly fine. but anything else is not.
When Edgar or leaper catches you, its instant game over, because there is no way to escape from it. and when brute spawns, its also instant game over, because there is no way one man can fight against brute, let alone killing it.
I'm not saying they shouldn't appear in singleplayer, but their spawn rate must be lowered significantly. in worst case, brute can spawn right after you start the round, dying instantly after the force field is gone.