Backstabing is *sort of* ok as bunch of mechanisms by which it's easy to get advantage on attacks have been removed. 5e Flanking rules are a bit of a mess so meh if it's different. The high ground / low ground advantage is a bit silly. It really should be cover which is a mechanism already available in the rules. It's also worth noting that there's a feat "Spell Sniper" which bypasses this mechanic, showing how (un)important it's meant to be.
