I can’t say that i’ve ever heard that as a company policy. I know that roving heads do cause issues with reduction in air flow depending on the length of tubing used and there are then issues with regards to the tubing between the pre-filter and main unit carrying contaminated air i.e. it will be contaminated internally. This creates issues about what to do with that tubing to make sure it doesn’t get used as the extract ducting, however these can all be factored in and dealt with. In my experience there will, from time to time, be areas within an enclosure where it’s just not possible to get decent air flow i.e. ‘dead spots’ so you do need to have some method for that problem otherwise it will be extremely difficult to get the clearance air tests to pass.
Really interested to see what anyone has experienced