Boiling method for DNA extraction helps in cell lysis but may be only partial. Combination of chemical and physical means may be more efficient for cell lysis as well as for extraction of good quality DNA which can be used for downstream analysis. Btw, I haven't used boiling method yet....only gone upto 70 deg to 75 deg C incubation along with chemicals for cell lysis.
Plz check the below links:
Article An Improved Method for High Quality Metagenomics DNA Extract...