They're honestly one of the reasons I dislike playing vanilla factions at all. Vanilla is already stronger than most sectorials, and if a faction has to have auto inlcude units, they should be faction specific, not generic mercs. I don't mind mercs as much as others, I just don't want them to be the go-to profile over faction-specific alternatives. And a regular, two wound, double-template CC monster for 14pts is worse than any of the other stuff people complain about these days imo.
Mercs getting sidelined for Vanilla should be the next thing in N4. We already have the system/button in place, just extend it to all mercs and call it a day. This should take Vanillas down a notch and allow sectorials to keep their specific mercs where they make sense thematically.
This is all well and good, but it detracts from the overall discussion about how Haqqislam functions at the moment as a competitive faction. Whether you like or dislike diggers, or mercs in general, they're making it into most (but not all) lists. And the more important point is simply that in the case of diggers they offer durable defence and regular orders for a very agreeable price.