I think flash cards can be fun and educational if you use them correctly. Introduce new flash card material with the 3 period lesson or math manipulatives. Make sure your child knows the concrete level of the material before introducing flash cards.
After successfully doing this, you can play different types of flash card games. I noticed my children made their own math facts cards after they understood the material.
Purchase or make 2 sets of reading flash cards so you can make Montessori 3 part cards.