The smaller ones tend to fit more naturally within a season. When they aren't trying to balance two dozen characters and the continuing storylines of four shows.
The Constantine/Arrow crossover was one of the very few Arrow S4 episodes that most fans liked.
There are also many smaller crossovers without the lead characters being involved that seem to work out well. For example: Felicity popping over to Flash from time to time, Joe West and Cisco showing up on Arrow to investigate Eobard Thawne, Diggle and Harbinger in the King Shark episode of Flash, etc.
Only problem is, at the moment very few characters can hop between earths. Barry, Cisco, Kara, Harry Wells, Jesse Quick, and Jay Garrick are the only heroes that have been proven to be able to do it (though we've been told the Waverider can do it too, just haven't been shown).
Cisco needs to make more of those gizmos he gave Kara. It'd be cool to see Earth-2 and Earth-3 characters jumping to Earth-38 and the other Earth-1 shows without Barry's direct involvement.