You mean giving anyone access to the data? Or open sourcing the code? If the latter, I think that's a generally a good practice. Security through obscurity is never good for public infrastructure. In this case, UK Biobank has now switched to a remote access platform (not particularly secure, as the data was found for sale on Alibaba today), but contracting it to DNAnexus and Amazon. Private companies have no incentives to open source data, unless mandated to do so.

In the EU, there is a bigger interest in building scalable but also secure platforms for health data. Hopefully good innovation will come from there.