Advantages-Ground clearance due to smaller ring (unless you run 'burns that is), generally a lighter setup than running a rear freehub, weight is distributed more evenly as no heavy mechanics on the rear wheel (Hope pro 2 is 427g + cassette or sprocket, Echo TR is 219g + sprocket), some will say they like the sound, generally cheaper.
Disadvantages-Some will say they hate the sound, chain permanantly moving so trousers get shredded very easily, no use for street trials because of chain turning constantly
That's all I can think of in terms of pros and cons, coming from oldskool trials like yourself I was a bit unsure to begin with but now I run FFW I wouldn't go back. I like the fact that you save quite a bit in terms of weight from the backend, cheaper to buy and maintain (buy an Echo SL, fit it and thats it) and I generally trust it more.
All it means is instead of having a freehub on the rear your hub has either a spline or a thread and a fixed rear sprocket on.
It's (yet again in trials) purely a personal preferance thing, people who run FFW's will swear they're miles better than freehubs and likewise people running freehubs swearing they're miles better than FFW's. If your in the situation where your gonna need to buy a new rear wheelbuild I'd recommend going the whole hog and getting set for a FFW, if you already have a Pro 2 or can get one cheap then stick with it as you'll never have to buy another, especially with Hopes unbelievable customer service.
Bit of a random answer but there you go!