In my experience, ropers are horrendous things to fight at any level. A calculated "Easy" or "Medium" encounter with ropers is never that. High AC, reach, and plenty of attacks make these guys extremely dangerous and hard to put down. With a bite that inflicts 4d8+4 damage, god help anyone who is on the receiving end of a critical hit. That said, I love ropers.
By extension, MToF has a new monster called the Lonely (under the Sorrowsworn entry) that has a similar attack routine with enormous reach. Granted, they are CR9 but these baddies are nasty with an excellent damage resistance and inflict continuous psychic damage. I sicked a couple of them on a 14th level party recently and it took the party everything they had to defeat them.