In terms of 'flexibility' IMHO what I find is that people who are saying that 5e is flexible then give a list of ways they find it flexible which is VERY narrow! That is, sure, within a narrow range of styles of play, you can go out online and find enough advice to fix the issues you ran into because you didn't play bog standard 5e. Outside of that, you're pretty much out of luck. I mean, obviously you can rebuild the game, but it is not particularly transparent and there is a strong 'baked in' set of assumptions about the core processes of play.Yes and no. The previous two editions really tried to lock things down into a specific style of play. There were rules for how to stealth, what DC it was to climb specific types of walls, rules for ... well they tried to make a rule for just about everything. So as a player you could always say "here on page XX is says that YY so you have to play that way". While the intent may have been admirable, it never really seemed to work very well.
Maybe there could be some kind of compromise, maybe not. That, and maybe I'm understanding what you're trying to say and maybe it's just been a long day and just ignore what I said.
I don't think that last bit is atypical of RPGs, but many ARE more transparent.
I feel like this all was kind of a major goal in designing 5e. The idea almost seems like "well, we will duck a lot of issues and make the core mechanical design decisions a bit opaque, and that will damp down all the debate we had with the last edition." It was a business decision, seems to have worked to a degree.
I do think it makes sense to mostly just play the game that is appropriate to what you're trying to do. THAT might be a great 'best practice', though it might seem a bit obvious to many...