The desktop app is electron junk that lacks basic features like changing spellchecking language and there's no pure web client like Whatsapp for instance. I also think it lacks some features of the mobile client like creating new groups and adding new contacts (or at least I haven't figured out how to do it). When I install the client on a new computer I can't access the history of the messages from the phone for a reason that eludes me.
Frankly 90% of my frustrations with Signal come from this client, the experience on mobile is better, although still far from perfect. I'm not a fan of pure web clients but web.whatsapp.com is lightyears ahead of Signal's electron bloated and under-featured mess.
The Android client crops the image when I take a picture through the app. No other application does that so I don't really know why it does it. I suspect it's a bug or maybe a bad setting I flipped by mistake but I can't figure out what's going on.
As far as I can tell when you get a new phone the only way to synchronize message history is to create a local backup on one phone, write down the ~24 digit passphrase, transfer the backup by yourself on the new phone then load it from there. If there's a simpler way I haven't found it.
Also I don't think you can move your "secret key" to the new phone, so you have to re-validate your safety number with them. Maybe there's a way to do all that but if I as a technical user wasn't able to figure it out while actively looking for it I have little hope for the average Facebooker.
There's also no support for WhatsApp-style stickers, URL preview, Youtube embedding and all these bells and whistles that I personally don't care for but I'm sure would be missed by many WhatsApp/messenger users.