LMDB is working well for me with Haven (personal relay). Apparently, some folks have had issues with it on underpowered VMs, and nostr:nprofile1qqsw9n8heusyq0el9f99tveg7r0rhcu9tznatuekxt764m78ymqu36cpz4mhxue69uhhyetvv9ujuat50phjummwv5hszymhwden5te0wahhgtn4w3ux7tn0dejj7qg4waehxw309an8yetwwvh82arcduhx7mn99uuwx66a also implemented support for BadgerDB (a KV store that powers DGraph).
I’ve been traumatised by MongoDB after years of building enterprise software. If you want something more robust and scalable, I’d go with PostgreSQL. Can't beat a fully-fledged relational DB with great tools and solid out-of-the-box JSON support.