Crappy may have been a bit strong but it depends what you are comparing it to. Japanese have far better plastics in general but thats German law for you
Comparing to the E34
Interior & exterior in general - whole alot more used in the E39 to start with and worse quality, recycled plastics for you.
Things like seat bases,door handles, shitty cup holders, interior trims, Cooling system- fan shrouds, expansion tanks, thermostat housings etc. Tap a E34 dash then a E39 haha.
Dont get me wrong I like E39's but quality wise they just aren't as good overall.