Nope, materials with crystalline structures don't dent... They crack/chip... and btw, even a diamond can crack/chip when hit by a rock at the right angle... It doesn't mean it's always easy to do so however... and if the units are based on organic technology then it matters that those units are older than human history... as that one unit may have just been defective or started to decay from lack of maintenance... Really, show me any man made object made of even the toughest known materials that can survive hundreds of thousands of years and remain in perfect working order?
While Apollon's armor is made of the same material and so far has not been damaged despite being in multiple battles.
In the Manga, when G1 and G3 met Imakarum for the first time and he blew them out the window, G1 fell all the way down from near the top of that Chronos Skyscraper to the sidewalk below and landed facing down... Now figure that G1 weighs 291kg and calculate how much force he landed with, never mind how hard he was hit by Imakarum... and both his body and CM remained intact... Even if he couldn't stand for a minute and needed Aptom to save him long enough to recover and go Gigantic.
G3 also got blasted again by Imakarum with a hand blast at point blank range... This is after he had already demonstrated he could easily destroy whole building and while full body singed and hurt enough to collapse to the ground G3 was regardless still intact...
Darzerb's napalm breath, as noted before is 3900 degrees, and the Guyver can't just shield the CM from fire that engulfs its whole body... So it can clearly take similar intense heat as the rest of the body with no threat of damage... Sorry but not even Titanium, which boils at 3560 K (3287 °C, 5949 °F), can take 3900 degrees but the CM material can...
For the armor, even Titanium only has a tensile strength of 434 MPa (Titanium alloy armor plates can raise this to 1,140 MPa, for Titanium ATSM Grade 38)... versus Spider Silk at 1.3 GPa... Lampet Teeth is 3 to 6.5GPa... and that may not be the upper limit to organic based materials as those are low density examples and the Guyver is denser than normal organic matter... So a organic based armor can be many many many times more durable than any present man made armor... So even as thin and light weight as the Guyver is, compared to a actual tank, the armor should be able to take an incredible amount of kinetic energy... Really, shrugging off impacts that would shatter steel is the kind of force that would turn a normal human wearing full body armor into a liquidated smear with a crater large enough to then bury what's left but the Guyver can take that practically all day long...
Mind, also that man made armor has limits like ductility vs tensile strength... Tank armor has to be flexible enough to take repeated strikes but that limits how strong it can be as the alloys we can make get more brittle as we make them stiffer and stronger and is one of the reasons why depleted uranium never gets used in armor designs...
But organic structures can be both strong and flexible, which is why they try so hard to replicate the properties of spider silk, etc. as materials that can share those properties can be made into armor far more effective than we have now... but even going against man made tanks it's usually preferable to have a 76mm, and 90mm anti-tank guns... 30mm is just the upper limit for small aerial vehicles... The A-10 actually slows down a few MPH as it's firing its 30mm cannon... and like you said, even that is only used because rounds that penetrate the armor can be used to kill the tank crew rather than actually take out the whole tank but the Guyver isn't that easy to kill...
So sure, not indestructible but clearly tougher than you're giving it credit for...