There is no such thing as "this flex is good for park/powder/slalom/GS/etc". If a 250 lbs person and a 120 lbs pound person are skiing park (at the same skill level), there is no way that same flex boot will work for both of them.
The appropriate flex is one that is based on your weight, ankle range of motion, some extent your height, and personal preference.
When you get a boot that (in addition to fitting well) has a flex that is selected based on the above criteria, then you will have a boot that most likely does never causes shinbang, skis great, handles slow jibs/butters, has support on big landings, and goes anywhere you want it to.
If you select your flex based on what someone else has or tells you to get (other than based on what is written above), then you will most likely have a shitty experience.
And for what it's worth, the entire Wells family, Durtschi, Benchetler, Sage, Gus, Elias, all ski on 130 flex boots, and Keefer & LSM are on 110/120 because they are simply a bit smaller/lighter dudes. Bottom line, don't think that softer is better and conversely, don't go overboard either. Get what is appropriate for you.