With Leon's current strength, transforming into his Flame Fiend form would cause all of his body's attributes to increase exponentially, making him much more powerful.
However, everything has two sides to them!
It was exceptionally hard to control the massive body of his Flame Fiend form, causing Leon's movement and attack speed to become severely reduced while he was transformed. The power of his attacks may have increased, but he was much slower. If he met agile opponents like Bleia in this form, his attacks wouldn't be able to hit their mark.
Then again, the Flame Fiend's combat style was more focused on defense and counters, and was therefore much less reliant on his speed.
