I have a gravel bike - Jamis Renegade Exploit. A great bike. But the rear derailleur Shimano 105 seems to be it's weak spot. I am suprised how easilly it simply fell off when it came across a rather small tree brunch. The weak spot is located near the place the derailleur is mounted to the frame. So the question is: can I buy more robust rear rerailleur for my bike (maybe from mtb groupset?) or 105 is the toughest I can get (it contains a mix of aluminum and steel whereas Ultegra is all aluminium which makes it even more fragile).
A MTB derailleur is definitely an option, but keep in mind you'll have to snag a Tanpan.
Honestly though, I think it might have just been a freak occurrence. In my experience 105's have been pretty good and hold up pretty well. You should be able to find another 5800 replacement for around $35.
Shimano road shifters will work as-is with any 9 speed era MTB derailleur. They're still easy to get - for example you can get the M772 Deore XT from a bunch of vendors on Amazon. There is one teeny catch to MTB derailleurs though - they usually don't have barrel adjusters on them anymore, so if you don't have one on your frame you might need to add an inline adjuster to your cable.
I would say worry about it after you break another one. I think it got hit just right, or possible had a material defect. You could send it in for them to examine and see what they say.
I agree with previous sentiments. About a year ago I suffered a bad over-shift error which sucked my entire Shimano 105 derailleur into my spokes (mangled just about everything)...a small part of the derailleur body did brake, but not at the location yours broke at.
Looks like a freak accident - they happen. Be glad the derailleur broke and not the dropout. Derailleurs are cheap, frames are spendy. The next one should last decades.
That result is almost inevitable if something solid (tree branch, thick mud, etc) gets sucked up by the chain into the idler pulley. I've seen it happen to Ultegra, Dura Ace, and various Sram derailleurs.
