Because we typically only ever see small groups of Predators hunting small groups of ill-prepared "prey" on foot. Could they do anything against, say, an Earthshaker barrage? What about Titans? How many of their ships would it take to take on an Imperial battlecruiser? Heck, even a destroyer is well over a kilometer long and armed to the teeth. You get the picture.
Their ships are small. Smaller than the DE's. But their home planet looks a lot like a cross between a craftworld and a hive city, yet surprisingly uninhabited.
Any predator vs Imperium conflict would be the predators going on jolly hunting sprees and dropping xenomorphs on everyone. Given the stealthy and potentially destructive nature of the predators, it'd be more likely to be Ordos Xenos vs Predators. You people expecting the predators to wage war on the Imperium are being silly. Just bomb them with facehuggers headcrab style.
...Though to be fair, the predators we see do go into combat half naked. In predators II I believe it was, one of the humans takes a plasma blast to the back and survives due to wearing scavenged predator armour. Judging from their big counterparts it appears they have a military/hunt leader caste.
They could win any battle against the imperium, but never win the war.
Gaunts Ghosts VS Predators.
Could they do anything against, say, an Earthshaker barrage? What about Titans? How many of their ships would it take to take on an Imperial battlecruiser? Heck, even a destroyer is well over a kilometer long and armed to the teeth. You get the picture.
Predators + Kroot dyson sphere.
The only actual experienced Predator who isn't going on a hunt to prve himself to his clan is ... that guy from AvP Requiem. I haven't seen it, but from what I hear it was pretty bad. Nevertheless, I also hear that (Wolf?) slaughtered Xenos like there was no tomorrow. So yeah, we've generally seen the Predator equivalent of teenagers becoming men or young 20somethings going on hunts to make themselves seem more manly.
I've no idea how good they are with spaceships and all that considering there isn't any lore at all on that from what I know. Like at all.
True about the success rate and the reprisal against the Imperium, but just the concept is awesome on it's own.
You know what I never got? How did that other predator know the killer catchphrase, "You are one ugly-" e.t.c.?
Was it the same one, or do they have some form of communication?
It seems to me that Predators would be interested in becoming even more buffed-up than they already are, regarding the reverse-engineering the Space Marine thing. I mean, look at the regular Marines - 8 feet tall, superhuman in every way, practically immortal, blah blah blah. Now take all that and apply it to a Predator. They'd probably be 10 feet tall, by my guesses.
Not compatible. If the predators didn't end up in an Inquistorial war or a scout war, it'd be most likely they'd only exist as fleeted glimpses in the dark, like bogeymen out to catch lone travelers.