Of course, the answer depends totally on which particular model(s) you're talking about. Some parts are done very well on one model, terribly on another.
As far as how many models are affected, I'd say unrealistically flat interior panels. Most cars, especially those older than the 80s, had window cranks, interior door handles, door lock pins, etc. that stood out from the surface. How many times have you seen a model with the interior panel details just sort of hinted at... or ignored altogether. It's especially obvious on convertibles, where the interior is the first thing you notice.
Harry, I would agree with you, to a point: Vitually all model car kits tooled prior to say, the early 1980's were done with "tub-style" interior units--not all that odd considering that until the "revival" of the hobby and the industry in the 1980's, the expected market for model car kits was who? Answer: Kids.
With a "tub" style interior "bucket", it's impossible to engrave high-relief details on the sidewalls, and get the part to come free of the tooling in demolding, and that is fact.
That said, as early as 1957, Revell's 1957 Ford Country Squire station wagon kit had a true "platform-style" interior, the floor of which was the floor pan molded en-bloc with the frame, with carpet and floor mat detail on the upper side; separate interior panels AND a separate inner panel for the lower tailgate, with quite nicely done raised details (door handles, window cranks, vent wing cranks, even the latch details for the inner tailgate panel. So, the knowledge of how to do well detailed interiors existed 55-yrs ago, well before the rather young model car builders we were back then were really capable (or desireous!) of handling more complex projects. Of course, also well to consider is that from 1958 (the first AMT 3in1 Annual Series customizing kits), kits of a given year's new cars were based on promotional models made for the auto industry, and as such had to be designed for rather rapid mass-production assembly lines, and that meant a lot of simplification--hence "interior tubs". This carried on with such as the first AMT Trophy Series kits, again made for the expected market, the (approximately) 10-16 yr old kid hooked on model cars.
However, with the recognition on the part of the model kit industry (lead in the US by Monogram) in the early 1980's, that a huge demographic shift was well underway, the "10-16 year old kid" suddenly having morphed to men in their 30's and older; it was time to rethink interior detailing in the design process--enter the return of the "platform" interior setup as pioneered way back in '57 with that Country Squire.
As for carburetors, if one looks at any real carburetor, there are raised details, and intricate linkages that are a part of every one of them, going all the way back to the beginnings of the automobile. Now, in order to create raised details on all 4 sides of a miniature carburetor, that will necessarily require tooling that will reproduce that, and that means tooling that will slide together, then slide apart, every cycle, in order to produce that carburetor. Think of it this way: Virtually EVERY one-piece model car body shell requires a mold with SIX sides (if you think about it, any cardboard shipping box has SIX sides: Right side, left side, front side, rear side, top side and bottom side. It's that way with a model car body, my friend: Right side, left side, front end, rear end, upper surfaces, and the core mold that makes the inner surfaces. To do a small part such as a carburetor in this fashion, the same would be true, especially if one expects a detailed carburetor throat along with all the proper detailing on each face of that carburetor. Now, what if the model kit has multiple carbs (think Chevy or Pontiac Tri-Power setups here), that's a total of 18 moveable steel sliding mold cores!
Now, how much are modelers willing to pay for all that? Hmmmm? And I haven't even addressed the intricacies of assembling such tiny plastic parts (Surely many here remember the carping about the overly intricate Trumpeter '60 Bonnevilles, the equally intricate Accurate Miniatures kits, no? Please note that I've not even addressed the really intricate parts of a carburetor--the linkages! Even if done in PE, those would intimidate a large majority of modelers, on two fronts: Extreme intricacy, and cost--that stuff ain't cheap folks!
The bottom line here is simply that some things which limited real surface detailing in the past (interior tubs) were actually pretty correct reads of the model kit consumer marketplace as it existed in decades past, while some of the limitations of what model companies are going to tool up today are those more related to costs VS returns.