I think part of the problem with some posts on here is that they have made a judgement about fast-track training courses without knowing anything about the trainee.
Having some relevant knowledge combined with transferable skills will make fast-track learning easier, e.g. someone with a background in any aspect of engineering moving into gas work. Plus, some people learn better in class-room situations than other people do, so what might be a disaster for some folk will be possible for others.
Private training providers might over-egg the benefits of fast-track, but then most marketing pushes the benefits of the product being sold, and leaves out any downside, so it falls to commonsense to look beyond the gloss.
The safest bet for most people will be to get some independent advice from a training or careers adviser, which might cost a couple of hundred quid, but if you are thinking of investing your time and money in a change of career, it's likely to be money well spent.
Fast-track is going to mean coping with a steep learning curve and finding opportunities to put theory into practice, but as some people on here have confirmed, it can be done if people have the right attitude and aptitude - and a big dollop of luck will no doubt help to make the wheels turn in the right direction.
I heard an interview many years ago with the guy who set up up Dewhursts the butchers (which for the younger generation was a national change of butchers) when asked about the secret of his success he said it was mainly down to three things: hard work; knowing the right people, and a lot of luck. Which I've always thought was a very sound and down-to-earth answer to a question that often draws an egocentric reply.
As lady-luck would have it, the supermarkets took over the sale of meat, and the Dewhurst chain of butcher shops was wiped out in the mid 90's, despite his hard work and knowing the right people. But his basic formula explained the downfall.