First off, while plenty of GMs will tell you if you have a penalty to a roll by no means will all those GMs tell you that penalty's specific numbers. "That wall looks slick, climbing it's going to be tougher than usual" is all they'll get, and not just from me.
I've had GMs who will say the number. I've had GMs who won't. I've had GMs who do both.
And, the wall being wet and slick might not necessarily give the same penalty every time, depending on other conditions e.g. a smooth rain-slick wall may well have a bigger penalty than a rough rain-slick wall.
There was a whole tangent on another thread exactly about this, whether or not a GM should have consistent numbers for this sort of thing. In this case, if the GM says "this is a rain slick wall" on one occasion and "this is a very smooth, rain slick wall," then different modifiers are fully acceptable. If the GM
doesn't describe it that way (or at least has that in their notes, but forgot to mention it) but has different penalties, it can be acceptable to call shenanigans, or at least be miffed due to lack of consistency.
But yes, I agree that those two things would probably have different modifiers.
But anyway, much of the rest of this doesn't really have much to do with why some people don't like roll-under systems; it's all about game design.
Sometimes characters just don't work out like you want them to. One of my main ones has the same problem - he can tank it up all day but his damage output is (by the party's standards) pathetic. He's a Cleric, which means his odds to hit aren't great, and his strength is a mighty 9. However, his AC is among the best in the party. So, when in melee he just sees it as his job to glue one or two opponents up by letting them try to beat him up until someone can come and bail him out.
OK, I don't quite think you get it here. I'm going to assume worst-case scenario here: you had your heart set on playing a cleric who could dish out large amounts of melee damage, but due to the way your game's chargen worked, you were stuck with a Strength of 9, and had to readjust, and are cool with the readjustment.
Here's the thing:
not everyone thinks like you. For a lot of people, getting stuck with a character they don't want is
terrible. Especially if they made up a background ahead of time,
which a lot of gamers do. Not everyone is into characters whose story emerges through play.
And that's why many people don't like roll-under systems, particularly when combined with games that are actively designed to produce low numbers to roll under.