If you let the bit do the cutting and don't force it through the work, you can consistently make single pass cuts without any extra strain on the router with a spiral bit. I wouldn't try it with a straight cut bit, you'll either burn it up, break it or both. Doing it in one pass will also make for a cleaner edge on the cut.
As far as the flush trim bit goes, 1/2" is a lot easier on the router motor and once again, as long as you let the bit do the work, single pass flush trims are not an issue. I've done full profile pattern cuts with a 1/2' flush bit.
