The analogue input and output sockets on the Mac are TRS. They will accept the Line Level signal from your guitar effects unit but you may experience level mismatch and ground loop issues. You may have no option but to govern the recorded level via the effects unit's output control. This, in turn, may adversely affect the sound.
Generally speaking, commercial I/O boxes are a good idea. There is the immediacy of physical controls. Signal levels get matched. The analogue-to-digital conversion is often superior to the computer's. Monitoring is easier.
Speaking for myself, the I/O box that I purchased in 2007 is still serving my needs today. The one thing that would force me to change it will be a computer with no backwards compatibility to FW400. i.e. A Mac Pro.