To put alltogether,
1/ IMO, very luck and dice dependent,
2/ Takes forever,
These two are very enough to make trench warfare unsuitable for clan arena i believe.
But you've notplayed these settings in a clan war you say?
Have a read of this thread. Lots of people have, including entire wars. It doesn't take forever, and rarely more than 20 rounds. You're just plain wrong there.
As for dice/drop - i can only assume that you don't play much trench? It's no worse than flat, esc spoils. Hit a rainbow early and you can roll over the board easily.