I read the first linked article where these experts in ocean plastic and ocean engineering supposedly give their great insights, back in 2015. It lists "6 Reasons That Floating Ocean Plastic Cleanup Gizmo is a Horrible Idea" (what a headline!). But the six reasons are either unhelpful, unconstructive, arguments in bad faith, silly, or exactly the sort of stop energy that activists specialize in. They were:
1. Ocean plastic doesn't behave the way the project's backers say it does.
Ocean Cleanup said the amount of plastic drops off exponentially fast after three meters of depth. The rebuttal of the expert oceanographers is that they stopped looking after five meters (as you'd expect if it drops off sharply after three), and that plastic can also be found at 100m of depth. This is presented as mistake by Ocean Cleanup but obviously isn't. Neither fact is incompatible with nearly all the plastic being in shallow depths.
Then they ding it because the machine as designed can't capture pieces of plastic smaller than 2cm, although lots of plastic has degraded to tiny particles. This is the nirvana fallacy in action: you aren't planning a perfect solution to the problem, therefore you're misleading people when you say you're going to tackle it. Only childish people think like this. Engineers know no solution is ever perfect.
2. The collectors will break really quickly
In 2004 the booms in the feasibility study weren't specced to a level that could survive large storms. It was designed to raise money and see if any plastic could be collected at all, so this is expected. Then it says Boyan Slat planned to do the 2015 test in the "relatively protected waters of the Korean Strait" i.e. somewhere large storms wouldn't necessarily be a problem. This isn't a critique.
3. The project will harm wildlife
By wildlife they mean plankton. This is a ridiculous and bad faith argument, the sort of critique that could apply to literally any object in the ocean including sperm whales.
4. Recyclers don't want the plastic
So? It can be landfilled. The goal is to get it out of the ocean, not create new Coke bottles. This critique makes no sense.
5. There's a far more effective way
They mean getting volunteers to pick trash off beaches. Very NGO mindset: why mechanize when you can hold a splashy single-day event and make everyone feel good about themselves. It was obvious why that doesn't work. The link they give to find the next event is a 404 and the NGO website containing it no longer mentions any beach cleanup events. Instead the "Take Action Against Plastics" section of their site now asks people to lobby Congress.
6. It's better to not put plastic in the ocean in the first place
Obviously. But people do, which is why the project exists. This is not a reason why trying to remove plastic from the ocean is a "horrible idea".
So: six arguments, of which the best is that the initial PoC wasn't built to production specifications - the sort of argument only people who have never built anything make. I don't see any reason to side against Slat here.