It was certificated - up to a point. Problematic? Maybe not, but it was a part of the flt envelope to be treated with respect.
Obviously there are no spoilers, and once you translate to 'vortex lift' (stalled in conventional terms) there is definitely no shortage of drag. (This happened at about 250kts at landing weight).
Supersonic - it was certainly no sailplane and an ability to increase drag wasn't required.
So - there is a bit of the flight envelope where you are subsonic, descending at about 350kts IAS, where you may need a bit of drag; e.g. to make the FL140 limit on the OCK 1A SID (as it then was) to LHR.
To facilitate this, engines 2 and 3 could be selected to reverse idle within certain strict limitations (most of which have now left my brain). The mechanism was to ask the SFE to arm the system on his panel and then to select reverse on the inboards. Where the system was slightly unreliable was that you were running the air-driven buckets with the engines at idle thrust - consequently they sometimes didn't make a full reverse selection, in which case you canx reverse on that engine and managed on one.
Clearly the big event would be if they didn't translate into fwd thrust, which is one of the reasons it wasn't done below 10 000'. I'm not aware of this happening.
To be honest it was only really used when ATC threw an alt constraint at you during the descent, because in general if you just pitched down to 380kts (Vmo when subsonic at typical approach weights) you would get the height off comfortably.