Well when Drake wrote these stories in the late 70s, IIRC ... the technology was still "new" so to speak. But Remember ... the USN uses LCACs today and has for years. And in SE Asia the US Army used some "Hovercraft" in the Delta Region in that Conflict, '62-'73. But regardless ... It all comes down to how far we are willing to "suspend our beliefs" in Sci-fi. And Slammer AFVs propulsion systems is far more "reasonable" then some of the "sillyness" that is G/W dogma ... I'm a big fan of having a modicum of reality in sci-fi ... I mean, many have heard my musings on the tactical deployment of 60 foot tall combat walkers ie., G/W's Titans, BT's Mechs, etc., etc. ... So IMO if you as going to allow 60 foot walking "Targets" on the sci-fi "battlefield" ... The Slammer AFVs are much more believable than that as well as other things that exist in the G/W 40K fluff ... IMO ...
But as David Drake pointed out in the afterword to the collection originally published as "Hammer's Slammers", he's a writer, not an engineer. And he was writing stories about an armored cav regiment, not developing new technology. Mostly, IIRC, he wanted to get away from the weaknesses of treads. So, IMO ... Titans, Hovercraft, etc., etc. ... pick your poison ...
But I can see by the response/lack of comments here, that I'm the only real "Hammer-phile" on this site !!!!