two things matter in fillets. size and density.
The larger the fillet, the more bonding area you have and your joint will be stronger.
The more dense you fillet, the less likely it is to compress and allow your parts to flex.
milled fibers are extremely strong, but they are also very heavy. Wood flour is an excellent filler for fillets because its relatively light weight and very strong.
The Gougeon Brothers book on boat building has some test data on fillet size vs. density. if your interested in seeing more.
