as I understand it assumes a set amount of carbs in a meal which I can't imagine a hospital managing well
Well, no. Sliding scale continuously adjusts insulin infusion rates based on BG which is measured every hour; if you eat a lot of carbs (note that it's frequently used alongside glucose IV) then BG goes up, is detected within an hour and the insulin infusion rate is adjusted to bring it back down.
Read all about it here.
There is nothing inherently wrong with the sliding scale regime that causes DKA (note that binging on carbs can't cause DKA to begin with - harmfully high BG, sure, but not DKA) and I suspect a failure to adequately monitor BG and/or to correctly apply the sliding scale rules.
I would have thought it would be better for any one that has to spend time in hospital to be in control of their own medication, but now they take it all away from you and make a right old mess of things.
The official recommendation is that patients should continue managing their own medication whenever possibly; however, in certain situations that may it be possible (e.g. NBM for surgery)