Can't comment on the garmin hardware compatability, but I have a Bryton 50 and rate it highly. saw me round France for 3 weeks without too many issues.
The good and also the bad, I got an HD map of UK and also full street mapping of Europe which is fine and doesn't need upgrading to HD imo as std it came pre-loaded with all of Europe POI. and all the EV routes ( haven't tried these for accuracy of routes yet!) Without upgrading the basic included EU mapping it does come with the std sat nav faux pas, as in it will send you down roads that used to exist, cart tracks etc To be fair these roads also appear on maps if they aren't completely up to date.
Battery life is 8-12 hours depending on your use of the backlight, the unit can be used whilst being charged. The charging point is on the rear of the unit so if you want to plug in an AA battery pack/ dyno hub/ solar panel, you will need to adapt the mounting (this is possibly).
At the moment you need to upload routes to Bryton sport before downloading them to the unit, this means an internet connection, however I believe thy are developing an app to allow this to be done standalone on your pc.
When transferring gpx routes from 3rd party planning sites you have no way of knowing if the route exceeds the maximum number of waypoints the unit accepts, the only clue is if the unit tells you the loaded ride is 0km, so best to re-plan the routes on Brytons own mapping to be sure.
Occasionally the unit will forget to bleep when you are approaching a turn (eg it thinks the road bends rather than turns of) so you have to keep an eye on the screen which can also look a bit cluttered particularly in towns) not a major hassle but worth noting.
