The principal feature that I consider when evaluating XMPP servers and clients is their support for reliable message delivery (no message loss on connection interruptions), and the main method of achieving it is with XEP-0198 ("Stream Management") [1]. Unfortunately Jackal does not support it, and AFAIK it is much harder to implement it for an existing server or client than to design the application with XEP-0198 in mind.

To support reliable message delivery as defined in XEP-0198, the application needs to:

(1) count and acknowledge stanzas;

(2) buffer unacknowledged stanzas for redelivery in case of stream resumption;

(3) resend delayed stanzas in case of stream not being resumed.

Note that some clients that claim support for XEP-0198 implement `urn:xmpp:sm:3` (Psi+, Conversations, Gajim with python-nbxmpp>=0.5.6), while others implement `urn:xmpp:sm:2` (Swift, Gajim with python-nbxmpp<0.5.6). The revisions differ in how they count stanzas. Ejabberd supports them both simultaneously.

The level of support also varies. An application may:

(1) just count and acknowledge stanzas;

(2) resend stanzas on stream resumption;

(3) resend stanzas on reconnect without stream resumption;

(4) resend stanzas after the application is restared.

Psi+ supports 1 and 2, does not support 4, and probably does not support 3. Swift supports only 1. [2]

[1] It is also technically possible for the client to use XEP-0313 ("Message Archive Management") to retrieve undelivered messages and to discover and resend unsent messages, but the latter is not described in the XEP, and I'm not aware of the clients that do so. I have found this suggested by Holger Weiß at https://github.com/redsolution/xabber-android/issues/128#iss... , together with a clever (currently abandoned) extension of XEP-0313 with `urn:xmpp:mam:sub` that subsumes XEP-0280 ("Message Carbons").

[2] To study interactions between Jabber clients and servers, I wrote https://github.com/orivej/xmpp-logging-proxy that serves as a Jabber server by relaying communications to the actual Jabber server while saving the decrypted streams to files.