XMPP is much more popular for private messaging, so you don’t have many large public group chats like on Discord (and lesser extend Matrix). It can do it, but clients are not really optimized for that to be honest.
As for the specific questions on e2ee: OMEMO as it is currently implemented in most clients is very similar to Signal in security, but like Signal it does not encrypt metadata. There is an updated OMEMO standard that does encrypt metadata as well, but it hasn’t been adopted by any popular XMPP clients yet. However both versions are significantly more secure than Matrix’s MegOLM, which has chosen to sacrifice a lot of security for user convenience IMHO.
XMPP is actively developed, but it doesn’t have much funding for the open-source efforts, so it lacks PR and some things don’t develop as quickly as what you might be used from VC funded for-profit companies like Element/matrix.
I like the Movim webclient, but most current users seem to prefer the native clients for XMPP.
XMPP uses way less resources because it was designed to scale to billions of users for chat, instead of being some over-engineered failed experiment to use a DACS for chat, which really isn’t a good idea and never was.