Now, I haven't played WoW in quite some time - since before the introduction of Mythic - but Extreme primals range, but most are what I remember would be on par to the first few bosses of any raid instance. A few mechanics and maybe one particular phase that might cause problems and you need to learn how to deal with, but overall fairly easy to understand and clear with a pug.
Savage modes have generally been the first two floors are easily clearable by a pug, but the last two have often ramped up in difficulty. I have heard it comparable to Mythic, but I have no experience to confirm or deny that. They are easily comparable to harder Heroic fights (especially the final floor of each tier) when that was the hardest possible difficulty though. Generally there will be multiple mechanics that outright kill the player and/or wipe the party should anyone fail, and this will be accompanied by tight DPS checks (healers will be expected to DPS along with their original duties, whereas you could get away with them solely healing in Extreme Primals, even if it is highly recommended that they contribute there too).