"less successful" is a correct phrase in written English. You can use it when you are comparing success levels between two or more subjects. Example: Despite his best efforts, John's new business venture was less successful than his last.
These were less successful.
Others were less successful.
Mitt Romney was less successful.
It's also vastly less successful.
Other movies were less successful.
Those I found less successful.
Other stories are less successful.
Awesome tool! I started using it one year ago and I never had to look for another app
Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia