Ive never done crossfit in my life .
I do agree that interval aerobics training is possibly number 1 way to lose weight
( technically working out is only 15 to 20% the battle ,it's all about what you eat )
kind of how crossfit uses , BUT crossfit
is not the answer to get your heart rate fluctuating , crossfit will do that but at the mercy of your joints, fascia, muscle ,and other connective tissue.
If you want to have longterm mechanical injuries or feel chronic soreness then crossfit is your answer .
You can do it safely by getting on a stair climber elliptical type machine and changing the speeds up and down throughout your workout or switch from one machine to the other without much break in between
My fave way is to swim . Zero impact on joints and every single part of your body is being worked