I've had a few close calls with printhead flooding as well, where I've waited and observed a good first layer, but some other problem like too much PVA glue on the bed has led to the part detaching mid-print. I'm especially wary now with printing with high-warp materials like ABS to ensure the first several layers print well and have good adhesion and bonding.
Obviously having the temp sensor fail mid-print isn't something you can help, so it's unfortunate you had to go to such lengths to fix the connection. I must've wasted just over a weekend troubleshooting my temp sensor problems as well and exhausted nearly everything in the process. Was contemplating ordering a new PT100 sensor and soldering it to the board, but in the end decided enough was enough and got the Ultimaker reseller to step in. Didn't want to void the warranty or risk ending up with a dead print bed. Fortunately I didn't have to pay a cent in getting it replaced, they turned around a replacement within days.
Since I started in this scene about 6 months ago, I've found a lot of the time spent learning has been with the complexities of FDM printing in general and less about Ultimaker specific issues. I accept that while some of the marketing can suggest 3d printing can be hassle-free, as with many things in life this may not apply when you start to push the envelope. For instance I've found printing with PLA and support materials really easy, but since venturing into more exotic materials like metal-infused / carbon fibre PLA, PC and ABS, you really do need to be willing to spend a lot of time in trial-and-error getting things right. And then there's the money as well - I've got the Bondtech DDG extruders installed, which enabled me to try out the CC Red 0.6 print core on my UM3. Works a treat, but wasn't cheap!
Anyway, I didn't really buy the UM3 in the hope that I'd get a so-called "plug-n-play" experience, I accept that's practically impossible to achieve outside the simplest of use cases. I bought it for its dual-extrusion capabilities and build quality - and aside from the heated bed issue, I haven't been disappointed.