When I'm fixing cross-browser bugs nowadays it's mostly about flexbox. It's not very often and I can always fix it in a small amount of time. But it gives me an unpleasant but also nostalgic flashback of the old days in the 00s where I fixed more CSS than I originally developed.
So yes, from time to time I run into different and not so obvious problems. My holy grail to work with flexbox today is this list:
https://github.com/philipwalton/flexbugs
Two recent issues I ran into: There is a very nasty "bug" in the current Firefox browser. It's not a real bug- more a different interpretation of the standard which will hit any developer not working primarily with Firefox I guess. It's about the intrinsic size of a flex item in combination with fluid images. You have to apply min-width: 0 to the flex-item or you get a (very) wrong sizing in some circumstances. See [2] and [3]
And another one. Most of the time I think about IE11 that it's a mostly 'modern' browser and therefore supports flexbox very good. Yes it is good enough to use. But don't underrate the flexbox bugs (just glimpse through [1]). For example you have to set `min-height > 1px` on a flex container to prevent it from collapsing in some scenario (holy grail layout is a very common case). See here [4]
[1] https://github.com/philipwalton/flexbugs
[2] https://bugzilla.mozilla.org/show_bug.cgi?id=1108514