I think an obvious example is Hangouts. It's easily the worst messaging app I've used in years. It consistently fails at reliably delivering messages. It's very common for me to send a message on my laptop, and then have the response only show up on my phone, or vise versa. The read marker will often jump around, and people that just messaged me 3 minutes ago will have "last seen 5 hours ago" under their name.
Google chat was a product that reliably sent messages to people, told me if someone was online, and was fast, too.
That's interesting, I have the exact opposite experience. I use Hangouts regularly, on two Macs, a PC and on my Android phone and I have never experienced any of that stuff, except maybe the "last seen" being out of sync - but that's hardly a dealbreaker.
I used Hangouts intensively for video daily meetings throughout a software project that spanned for several months - on different machines (on my work laptop, mobile phone, sometimes my home PC).
These meetings weren't big - typically 3 to 5 people - but when it worked for everyone at once, without reconnecting, rebooting etc., we cheered. This was an unusual occurence.
I had Hangouts crash - especially on Android - I had it request a browser plugin to be installed over and over again, and I never managed to get it to work on Firefox (switching to a different browser for Hangouts only proved easier), I had no audio, or distorted audio, or people dropping out of calls...
I'm glad your user experience wasn't as abysmal as ours, or rcheu's, but in my opinion you're a lucky one
One good feature it has is that it recognizes keyboard tapping, and suggests you put yourself on mute when it's detected
I don't mind the presence inconsistencies, & the read marker only rarely "desyncs" for me, but the notifications are absolutely horrible. I chat on one device and replies trigger notifications on 3...drives me nuts sometimes.
Google chat was a product that reliably sent messages to people, told me if someone was online, and was fast, too.