From what I seen until now, San Myshuno Festivals don't work so well with Seasons, Geekcon took the biggest hit, I think that electronics based festival should have canvas above the electronics in case of rain, however, I know that it can be hard to implement, so another solution is making the electronics unbreakable, like the 3D gaming rigs and DJ booths from Get Together, which you can't use in the rain, but it won't break and you can use them safely after the rain is over.
Another thing is the plants in Spice Festival and Romance Festival, the festival organizer shouldn't plant seeds that won't grow on the season that the festival take place, this also have 2 solutions I can think of, one is that the festivals will only have plants that grow on the season, and the second is that those plants grew on a greenhouse before moved to the festival, so they can still be harvested outside of season.
The Sims 4 had great job so far on making different packs work well with each other, and by making the festivals work better with Seasons it will be ever better! speaking of packs working together, I have other very minor suggestion, if you have Movie Hangout and Seasons, I think that horror films should be considered as "Spooky Spirit" holiday tradition, and romance films as "Romantic Spirit" or "Watch Romantic TV", films are great way to celebrate holidays in real life after all!
While I haven't heard as much upset over the Spice and Romance Festivals plants often being out of season, I totally agree with you on that as well! 
I hope they are working on these issues ! 
