In trying to answer your questions will only deal with blocks that are found on the average cruising sailboat rather than delve into the hardware that is found on racing yachts and multihulls.
Here is a short list of the block types that could be found on a cruising sailboat of around 37ft.
2 Speed Mainsheet Systems
Roller Ball Bearing
And the most useful block to have when one of the above fails - is a 'Snatch Block'
Here is picture of a Ronstan RF 6841
* Removable spinnaker sheet deflector blocks for boats up to 13m (43ft)
* Temporary sheeting point while changing sheets for boats up to 13m (43ft)
* Temporary reefing block for boats up to 13m (43ft)
* Spare or replacement block for general use on larger boats depending on line angle and load
* Temporary outboard sheet lead for headsail trimming
* Grade 316 Stainless Steel frame, load strap sheave and needle roller bearings
* Soft, thermoplastic rubber cheeks
* Investment cast Grade 15-5PH Stainless Steel snap shackle
By the way, blocks do fail - usually because they have been installed where the loads and angles are too great. On racing sailboats and multihulls - blocks will explode when overloaded.