I'd connected a mx 204 10 G interface with a DAC cable.
Doing ok for some weeks. Are there any drawbacks? Can I continue using this way or is better to use a SPF+ module?
The official hardware compatibility guide doesn't mention DAC cables - only fiber transceivers; https://apps.juniper.net/hct/product/#prd=MX204.
That said, I would not expect any issue by using a DAC for short reach. JTAC can potentially request you to replace with a fiber transceiver in case of interface link issues.