It's lowering barriers to trade, making it easier for goods made in one country to be sold in another. One way it does that is by harmonizing regulations, so you don't need to make different versions for different countries, or pay for lawyers to explain compliance laws separately for each country.
Only part of that results in lower barriers to trade. Mostly that reads like "follow these rules, update your processes and factories to these other specifications" - significant barriers themselves, just upstream from the actual trade.
More significant is that there is little transparency and a, seemingly, active avoidance of public discourse about the rule-setting. The rule-setting is where the power lies, and that's what most of the objection is about.
