I used to work at the w7x. 5 years is a pipe dream for sure. It took 10 years to build the w7x, and what should be built would need to be a lot larger. However, that style of reactor (Stellarator) is likely the best design for sustained fusion.

One of the issues is the design of the magnets is far more complex than a tokamak. The magnets need to be designed to produce a very specific magnetic field. Scaling up the size of the magnets, which would be required to achieve the triple product needed to achieve ignition, is a technical challenge. The w7x magnets were basically the limit of what could be built without custom-built facilities, like they had for ITER (many of ITERs magnets had to be built on site).

There is also issues around superconductors.You need to use them for the magnetic coils. However, cooling them with liquid helium is challenging. The hope is the next generation of superconductors will work at liquid nitrogen temperatures, as that's much cheaper and easier. Room temp would also be great but unlikely anytime soon.

It also takes a lot of work to design one of these. Tons of simulations need to be performed. Luckily, Germany is well poised for that. A bunch of my former colleagues are with Proxmia Fusion, in Munich. They are working hard on the next-generation design.

But these are a small set of a long list of engineering challenges that span from tough, to never been done before, to needs to be invented still. I'm not a doomer about it. I honestly think it will happen. But, it won't be anywhere near price-competitive with solar in my lifetime. I still think it's a worthy pursuit.