For standard aerobic and anaerobic conditioning within CrossFit, nothing beats the standard metcon.
There is no need to reinvent the wheel here. Couplets, triplets, and variations off of those simple templates are used daily to push heart rate and work capacity within our athletes.
With the exception of Saturday partner/fun WODs*, weekly WODs* are typically short and fast. Long, drawn out metcons tend to involve large numbers of reps, which tend to lead to a breakdown in form, which tends to lead to an increase in injuries.
CrossFit in and of itself is not dangerous. Dangerous CrossFit is dangerous. Metcons are programmed on 4- and 6-week cycles, with an emphasis placed on a particular skill. For example, a 4-week cycle may focus on developing the strength to perform one-legged squats (pistols).
Metcons within that cycle may have an abundance of air squats, deck squats, assisted pistols, etc. to develop the muscles and neuromuscular proficiency needed to be comfortable squatting on one leg.
* Click to learn some CrossFit Acronyms