There isn't really any sort of solid consensus. The biggest problem is that Gildas and Bede (who drew extensively from Gildas) are the only literary sources that are roughly contemporous with the events of the migration. So there's no other accounts to compare Gildas's sermon to or any that disprove what he says. However we cannot take his assessment at face value because of the religious approach that Gildas takes in his explanation. The arrival of the Saxons and their success is due to God's will and punishment, not the happenings of self motivated actors.

Now this doesn't disqualify him from having the general gist of events down. However we should not categorize this as a mercenary rebellion. Neither Bede nor Gildas indicate that the mercenaries rebelled because of a lack of pay, and we cannot even know for sure if the groups that attacked Britannia were the same as the mercenary groups. It is possible but unverifiable. Peter Heather argues that the mercenaries and raiders were two sides of the same coin, relatively small war bands seeking to make their name and acquire wealth, coming from the Roman periphery into the Roman world.

We do not know the identities of the leaders who led the "invasion" and Bede and Gildas agree that it started off pretty small. Horsa and Hengist only bring three ships after all. At the same time, we cannot discount it either due to the lack of alternative sources.

In the end short of new accounts being brought to light, there's not a whole lot to go on in regards to his accuracy. We can only work with it and keep in mind the biases that Gildas had and the axe he wanted to grind.