Kirin's Lightning Bolts are an unblockable attack, similar to Rathalos' Talon Grab, Anjanath's munch. You need a Guard Up Decoration, or the 3 piece Uragaan set bonus to successfully block those. Teostra's Supernova falls in this category as well, and I *believe* Beetlejuice's turd nuggets count too, but I can't recall. Vaal Hazak also has 2 or 3 unblockable attacks - his Bad Breath stream that he likes to swipe across the whole battlefield, and the one where he stands up on his hind legs, and crashes down HARD. Not sure about Kushala, she's just a cunt and I'm not looking forward to fighting her with only 3 Flash Pod blasts before she's immune. I stopped playing on the ps4 before Kulve Tarorth, but she has an unblockable attack I think. Deviljho's breath is also considered unblockable, and I do believe he has an unblockable attack that is kinda like Anjanath, but my memory is foggy.
Nergi's divebomb can be blocked without it, but hits so hard that you take significant chip damage, even with Guard 5. Do NOT try to Counter Thrust it, because CT blocks a single hit - Dive Bomb typically hits your shield 3 times because he ejects his spikes as projectiles when he lands. Using Power Guard will prevent getting knocked back during these special attacks, but you'll take more chip damage, since its damage reduction does not scale with the Guard skill like Counter Thrust, Guard Advance, and your basic R2 shield block.
Regarding Radobaan, Charge Blade is the one shield that is more effective against him. With a Charged shield, a properly timed Guard Point will disrupt his roll, and possibly even KO him. Plus, he has so much breakable shit that a single SAED decimates his bones, and opens up weak points for the whole team.
The shield is such a big part of the Lance, and it's a relatively low damage weapon, I don't see why it doesn't get more than just a shitty shield bash for damage. It's my one gripe with the weapon. After using it extensively, I honestly find it hard to use a different weapon anymore because it's the one weapon that actually makes me feel like I'm overpowering the monster by shrugging off everything it throws at me. Plus, I'm lazy and like to stand in one spot.