[Following are my interpretations only.]
(1) In some cases “shit-” can be taken to mean “shit”.
My car is a shitheap. = My car is a heap of shit.
I live in a shithole. = I live in a shitty hole.
(2) In some other cases “shit-” seems to mean approximately nothing but (I suppose) adds emphasis.
He had a shitfit. = He had a [real/big] fit. [Fit = ‘emotional outburst’ or so]
That’s shit-hot. = That’s [really] hot. [Hot = ‘excellent’/’great’ or so]
He has a shitload of money. = He has a [real/big] load of money. [Load = ‘large quantity’]
(3) I’m not sure about this one:
I ran into a shitstorm. ?= I ran into a [real/big] storm. OR ?= I ran into a storm of shit. [Storm = ‘violent/messy situation’ or so]
I suppose the dictionary should show both applications of “shit-”.
Note that the examples in group (1) above can also be construed as in group (2).