How sensitive are all-metal print heads?
Full definite answers on your questions are hard be given, there are a few parameters affecting the clogging of all-metal hotends.

All-metal hotends are considered so-called "upgrades" for printers while in fact they usually cause more problems. For high temperature filaments, like PC and some Nylons, etc. (printing temperature above 250&nbsp;&deg;C)  you require a hotend that can withstand higher temperatures for prolonged use of the hotend. The problem is that heat creep (heat climbing up the throat/heat break) causes low temperature melting filaments to become prematurely soft and cause problems when retracted.

This requires the printer operator to provide enough cooling air to the coldend of the hotend assembly (usually the part where the cooling fins are) to dissipate heat creeping up the heat break and carefully tune printing temperature and retraction length.

Note that Ultimaker Cura uses quite large retraction values (default 6.5&nbsp;mm), which work perfectly for the UM3E. This could be closely related to the compression effect of the filament in the Bowden tube. The all-metal hotends from the UM3 are perfectly capable of printing low temperature filaments, so if dialed in correctly, such hotends are not only to be used for high temperature filaments.