A lot of very useful statistical models can't benefit from the GPU, and for them I think Stan is the better tool. There's a reason it's basically the default choice for people who want to use probabilistic programming for Bayesian statistics.
It's probably not the best tool for AI/ML type models. But for statisticians who want to use Bayesian methods it's close to perfect.
Haha I knew "legacy" would ruffle some feathers, by which I mean Stan was pretty much the first application-ready PPS on the block -- robust toolbox of methods, actively developed/supported.
The last slide of this presentation on SMC inference in PPLs is a nice view of the PPS landscape (not to mention the whole deck is a great intro by Lawrence Murray): http://www.it.uu.se/research/systems_and_control/education/2...
I've never actually used BUGS or JAGS for anything but I would consider at least BUGS to truly be legacy software. To me the word "legacy" means you shouldn't pick it for a new project.