yes, but that is trivial enough, and once you add bonuses the roll under ‘falls apart’, either you have to subtract the bonus from the roll (addition is easier than subtraction) or you have to add it to the value you need to stay under rather than the roll, and that is a little weird.
I wouldn't be surprised if the primary disconnect comes from rolling multiple dice under a value that doesn't map to an easily intuitable value. Ever play Call of Cthulhu? The percentile values always struck me as very easy to interpret since an Anthropology score of 70% means 70% chance of success on the die roll and it translates to being knowledgeable about 70% of the field of anthropology (for the setting time period - By Gaslight, 1920s, Now).
Yes, more big number more better. It also feels like it's more easy to obscure difficulty without off-loading work on the players. "I rolled a 23, do I succeed" is easier on the player than "I rolled 5 below my skill, do I succeed?" (because the latter involves the player calculating margin of success), so there's more of an assumption that the GM will tell you modifiers up front in roll-under systems.
I would agree that comparison is faster in general, but number ranges also matter. At least personally, I feel that addition and comparison are both relatively fast for single digit numbers and a moderate amount of dice (I would place the limit lower for addition than comparison - probably 3 vs. 10 dice).
I agree with your general assessment, but one option I find more intuitive is to use two thresholds in such cases, i.e. difficulty as the lower bound and skill as the upper bound (e.g. Whitehack does something like this for AC in combat).
Did your GM not use situational modifiers? That's how you reflect the difficulty of a situation in GURPS, just as you set DCs in d20 systems.
The Troubleshooters has a neat way of doing difficulty in a roll-under system. If something is easy, you roll at "+N pips", and if it's hard you roll at "-N pips". If you're rolling at +N pips, any ones roll of 1 through N is a success, regardless of your skill, and if you're rolling at -N pips any ones roll of 1 through N is a failure regardless of skill (so if you have skill 69 and are rolling at +2 pips, you would succeed at 01-69, 71-72, 81-82, and 91-92).
