Given class Foo and Bar. Is having a many-to-many in one equivalent to having many-to-one in both? I find the latter one easier to cope with.

definitely not! these are totally different situations. using only many-to-one mappings you can only get access to one related entity. you need a many-to-many or one-to-many to get a related collection of entities.

Is many-to-many in one equivalent to having one-to-many in both, then?

Many-to-many cannot be equivalent to one-to-many because even if relation cannot be navigated, you can access to relation via HQL

So having a (A) one-to-many (B) (and many-to-one on the other side) means that for 1 A there are many B AND for 1 B there is only one A

So having a one-to-many (or many-to-one) both side will lead to one-to-one or inconsistence.

The equivalent of many-to-many can be done by A one-to-many AB many-to-one B, but you need an AB object.