We’ve had one of these for more than 3 years. It’s back in regular use with our second kid now and still think it’s great. It’s been into the hills and things a fair few times and is comfortable for a good few hours. More comfortable front in facing than front out facing, but both comfortable enough. We found over about 9-12mths they started to get a bit heavy for front if carrying for more than an hour or so (leading to aching back), probably not related to the sling. You can have them front facing in or out, or spin the sling around and carry them on your back with this sling though so can mix things up. We generally used a different back carrier (Osprey back pack) more than this for longer walks once they were ~9mths+, but the sling was good for back carrying when we went away without much space for stuff.
I use this sling on the front and wear a normal day bag on the back quite a lot and find that fine. Sometimes get in a bit of tangle when taking on/off but other than that ok. Balances the weight out a bit too.
We found a local sling library and gave a few a go before settling on this one. Guess most sling libraries won’t be operating at the moment though..!
https://ergobaby.co.uk/carbon-grey-cool-air-mesh-bundle-of-joy