There is a maximum chest level. If you go into Guild->Seals, you will see on the right hand side what levels of cards are available from the guild keys.
Guild chests have a 50% chance to drop the special common guild guardian troops.
The other troops are still random, but their rarity can be increased by the whole guild earning seals. At the top level 40,000 seals, the keys are similar to gem keys, in that they cannot drop regular commons or rares.