Client want me to use a swivel joint (www.Rotaflow.com)
in a 8 bar relief system.

Whilst my better instincts say no, i struggle to
be able to offer firm reasons why.(Vibrations/cleaning/maintanance)

The swivel joint is downstream of the burst disk/elbow to enable disk removal

I'm fully away of the industry norms here(jackscrews etc) but would appreciate any comments good or bad about this swivel joint.

Not a listed component in ASME speak what credentials do they carry????? The web site seems to be as thin as I wish I were......

Is this one of those famous 'Moment Filters'?

No absolutely not... ASME B31 codes have listed standards. Piping components manufactured to those standards are known as "listed" components.

This listing means that they carry pressure and temperature ratings,

The Rotajoint does not IMHO opinion conform to a listed standard, therefore it is subject to the requirements in the codes for unlisted components. The manufacturere should provide written certification that it complies with these requirements otherwise their use in a B31 system puts it out of compliance.

Now if your code jurisdiction is another code you may find similiar requirements in it.