How reliable are free sources for historical data?

Has anyone used free sources like histdata.com, dukascopy, forexsb to train models or backtest their algos?

I am working on training a model to trade forex pairs. Still experimenting with the idea and don’t want to invest in an data api before I have a solid strategy.

Wondering if I can make use of this data to test the model with this data and move on to test the model with something paid afterwards OR just go with a single data source as best practices would say.

Thanks in advance!