The 30L-1 I have was bought by me many, many years ago from a good, reputable amateur radio store. (Amateur Radio Center here in Baltimore) One of the RCA 811A's had a hole burned in the plate, a couple of the others had noticable "hot spots" in their plates. It came from a local ham that was a "Collins Conniseur". It still has the same tubes in it to this day, and still makes full outpoot!
Which goes to prove that they are not hard to burn holes in the plates if one is not careful when tuning up.
That old 833-A that had been kicked round the shack for years, with the huge shiny spots on both sides of the plate structure and the sunk in spot on the glass where a hole was almost melted in the envelope, is one of the best modulator tubes I have ever used. Before I installed it in the Gates, I could just barely make 100% positive modulation. With that tube in place, I can now modulate up to about 135% positive.
Normally a BC1-T wouldn't be capable of modulating up that high under the best of circumstances, so in case anyone might be wondering, when I converted the transmitter I replaced the big plate voltage rheostat with some fixed wirewound resistors to drop the PA plate voltage a few hundred volts below that of the modulator plates.
Regarding the grounded grid amplifier, some of the driving power passes through the tube and appears at the output at all times, because the input and output circuits share a common load impedance from cathode to ground. But I have never heard anything about it not being possible to overdrive a G-G linear. On the contrary, just listen across any band to the slopbuckets who are running amplifiers. I'd say you can overdrive a G-G amp and cause splatter, even damage the tube if you get too carried away. Even though most of the driving power might bypass the tube, if you have enough total drive to begin with, I'd say you could still have enough left to trash the band and possibly the tube.
Most likely what saves tubes in many such cases is that the ricebox itself is capable of only so much output and flat-tops beyond a certain point. This trashes up the band but spares the grid of the tube. Splatter on the bands probably results as much from flat-topping in overdriven exciters as from overdriven leen-yars.