It all comes down to the prop on the failed being secured.
On one engine with the prop secured your down to 40-45% effective power performance.
If your zero thrust torque is 15-20% on an engine to simulate feathered and the engine isn't feather that then brings you down to 20-25%% effective power. Which just isn't enough to keep you S & L and speed stable.
Its why we have NTS and auto feather in TP's if you don't get rid of the drag of the failed prop you have the flights dynamics of a brick.
There are a few types which are not in that situation which is the SAAB 2K and Q400, from mates that fly them they are that well endowed in the engine dept even at a full load they have to be careful of overspeeding on single engine.
The SAAB 2K on a single engine a FL250 will quite happily overspeed.
Some of us have to deal with drift down alts in the region of 5-6k ft single engine and a full load.