There have been multiple topics regarding this question. Also a youtube channel 'called Cybord4Life' has answered that question as well.
To not waste your time further here is the detailed answer:
Femurs ARE BETTER than tibia lengthening generally
Why? Because they can be lengthened more safely, cost less and heal faster.
The only disadvantage are proportions. If you stick to 5cm it's fine. But if you go for the full 8 cm, it's highly recommended you go for tibias too to balance things out. Most short people pull off the 8+4/5 very well.
Now if you are one of those people with extremely short femurs compared to tibias then yes u cna go full 8 without caring about proportions.Try making mock-ups so you see yourself.
A thing to keep in mind is that femurs are the longest bone of your body, hence the one providing most height. 90% of people i personally see in my daily life have long femurs. That's why 8+4 is well fitted for short people.