For Session:
Accounts can only be used on a single device, multi-device support is planned for the future
Can confirm that this is not true. I have multi device set up and it works just fine.
Files are temporarily stored on a central server (encrypted) until the recipients retrieve them
Not just files, but also your messages. However, it's not as "centralized" as you think. The messages are stored across the Loki network, not just on one centralized server.
Still new, bugs exists and features may change
Another con I would add is that it is painfully slow. There's often a delay of 10-15 or more seconds between sending a text message and receiving it. Interestingly, media files have about the same "lag". This makes faster conversations difficult, since I would send a message and an older message would backfill in making my message irrelevant. For example, I would receive "I have an idea", send back "what is the idea", but then a few seconds later a message would backfill in and appear before the message I just sent with clarification on the idea.
But that being said, I still think Session is one of the best truly-secure messengers out there. It's bug-free enough for daily use, very decentralized, solves the "offline message" issue. The only concern I have left is their weird crypto integration. The nodes in the Loki network are crypto nodes and it requires staking some $LOKI to join the pool. You do get rewarded for participating in some ways, however.