I have the advantage of my current dealings are all for my own interest
In the past I have bought a fair number of the NASM films but purely ones relating to aircraft blueprints and engineering processes so locating which ones cover the material is not so difficult. Sometimes it meant buying 20-30 rolls which was expensive but if I needed them I got them. I had my own A1 and A4 reader/printers so third party printing was not a problem.
I wish the NASM would take a leaf out of AFHRAs book and convert all the film to digital. It is far more user friendly for everyone concerned and prevents further damage to the original film. Fortunately there are a number of websites now that have done the conversions and you can download the pages.
The NASM does have a very detailed catalog of manuals for various aircraft and components and those were always provided as good quality photocopies. For the P-40 for instance the old index is many pages long containing 9 entries per page. More recent catalog printouts are 5 manuals per page and include the price of each manual.
AFHRA are great as they have converted most, maybe all, of their microfilm to PDF but the minor problem there is the index pages do not reflect the PDF pages because when converting to PDF they did not include the blank pages and sometimes the film frame numbers are missing*. That can be inconvenient but is still far better than having no sources at all. I have only found one set of errors in their catalog and that relates to General Kenneys diaries where the person back in the 50s who created the index for the specific microfilm screwed up. I can't blame AFHRA for that.
* The AFRHA microfilms had the index of contents in the last few frames and they photographed both the front and back of every sheet. Deleting the blank pages can throw the index out considerably so for some I created my own index which I shared with others. I only indexed items of interest to me and added notes.
I have not dealt with the USAF museum or AHEC for years but they were always excellent.
St Louis were slow but I never had a problem there.
My dealings with the PRO before the name change to National Archives at Kew were always easy and productive. Back then of coarse I was dealing with a person, not a computer, and asking for a specific document, usually an Air Publication, so that probably explains the difference from what you describe above.
Australian Archives are a mess. If you find a miss indexed item and ask for a clarification you seldom get a reply. I asked why an item would not download on Jun 3 and there auto-response says that
You book items for viewing a month in advance and travel 1,500km and the items are not available. What also really peeves me is the 300 km leg from Bundy to Brisbane costs double the 3 times further Brisbane Sydney sector and then the Sydney Canberra sector is often a hassle for some strange reason.
They tell you that you must not bend pages to photograph them so you pay them to "professionally" copy them for a ridiculous fee and the result is badly bent pages photographed under shit lighting. @#$%^&*