Wind wasn't really a hard problem to crack.
They screwed up the bearings on the first one in Orkney, did a bit of vibrational analysis and basically everything was cracked.
Tidal is orders of magnitude harder to engineer.
One of the big things is getting the plant to stay in the same place.
The forces involved are hurenous because you dealing with a liquid.
But because there are huge forces there is loads of energy available but when things go wrong it tends to destroy everything.
It doesn't help matters though that there is so much crap floating around in sea water. Then you add in corrosion etc and you have quite a difficult engineering problem to crack.
The media used to take an interest in wave and tidal. Its just they have turned up so many times to get soaked while the boffins fanny around trying to get it to work. Then nearly always it sinks or floats off or just disappears when its rips its anchor lines.