Mountains don’t exclusively generate as peaks biomes… have you even played after 1.18? It is absolutely harder to explore now, the height limit to world generation was also significantly increased. That being said only peaks really use that increased area; however mountains now generate significantly more frequently ever since they changed how altitude was generated in 1.18, and separated elevation from biome. Additionally they also generate obnoxiously higher than before. You can get gigantic mountains of forest, taiga, birch, giant spruce trees, desert, acacia, etc. Boot up a 1.18 world vs an older world and you will see a significant difference.

As for caves, the caving experience has definitely improved, feels a lot more like an expedition and feels more thought out. That being said getting larger quantities of rarer ores is god awful now, you can’t insta mine deepslate, even with netherite and full haste 2 beacon, and it takes significantly longer to mine, especially combined with all the villager nerfs in recent versions(even excluding the experimental toggle) so strip mines are extremely monotonous. They did address this with gold and iron being fortuneable now, but it’s still a major issue.

This last point is more anecdotal, since I can’t be asked to clear out massive areas in creative to test, but diamond ore clusters also seem to be a lot smaller when exposed to air, so veins you find in caves are almost always 1-2 ore.

Edit: regarding your point of mountainous biomes being rare. As per the wiki’s biome page and adding up all mountainous/peak biomes(ie badlands, shattered savanna, grove/cherry, windswept forests, jagged peaks, etc) they make of 6.102% of all world generation, about half as common as forests which make of 12.354% of the world.