The downside is that 256QAM is more susceptible to noise, and hence requires better SNR and signal strength.
For a 256QAM modulated downstream channel, shoot for SNR above 33dB. Anything below may cause connectivity problems.
Thanks to oldTDNickell5
*This FAQ is based on user knowledge from a volunteer core of BroadbandReports' members. This FAQ in no way constitutes official information from Comcast or any of its affiliates.