With the honourable exception of the last two posts, there has been some serious rubbish posted in response to the OP.
There may be a handful of grumpy Captains who whinge about flying with contract co-pilots, but the same individuals complain about everything else as well. This is the case in every company. But the vast majority are as professional as any pilots you will encounter anywhere in the world.
As for contract co-pilots breaking a strike. With what Captains?
And regardless, IALPA doesn't do strikes, at least not its own strikes.