It is still risky to do. An enraged decapitated flehspounder can still kill, or seriously hurt, you during his 5 seconds of headless berserking. Atleast on hard/suicidal.
Yeah. If he's got friends your mates need to keep them off your back. Otherwise they'll pin you down and either they or the headless FP will kill you before you can retreat.
Hell, even on Normal if you don't get away fast enough or your mates don't finish him off, the headless FP can and will kill you dead.
Everyone knows you shoot a bloat in the mouth with a LAR or two and it goes poof, scrakes can be chain stunned with powerful enough weapons (if you think it's a glitch, look at how gorefeasts and clots react to an axe or machete power hit they survive, they get stunned (and those would be their weaknesses if you even bother to need em).
And hell, most of that (save the Scrake stun) happens with the bullpup too. I've stunned Gorefasts with headshots that turned into bodyshots, and I regularly pop bloat's heads with a three round burst to neutralize their vomit as quickly as possible.