Sorry to hear you're having trouble with FreeCAD crashing – it's OK with me even though I'm using the very latest version:
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OS: Ubuntu Core 20 (ubuntu:GNOME/ubuntu)
Word size of FreeCAD: 64-bit
Version: 0.20.29177 (Git) Snap 66
Build type: Release
Branch: tag: 0.20
Hash: 68e337670e227889217652ddac593c93b5e8dc94
Python 3.8.10, Qt 5.15.3, Coin 4.0.0, Vtk 7.1.1, OCC 7.6.2
Locale: English/United Kingdom (en_GB)
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Which version do you have? At the moment, the one in the standard Ubuntu Repository is out-of-date and I had problems with usually good PPA version: seems FreeCAD are moving away from apt to snap. (The FreeCAD snap is available from the Ubuntu Software Store, or can be installed on other Linux flavours from the command line). It's the up to date production version and stable (in so far as I've tested it.)
I share your pain though! I'm learning Solid Edge and keep crashing it, much worse than FreeCAD! I have strong reasons for suspecting operator error, a mix of under and over constrained sketches, faulty 3D edits, assembly relationship errors, and repeatedly pressing the wrong buttons or the right buttons in the wrong order whilst trying to suss out how stuff works. Basically creating a muddle so confused that the program gives up!
Same crashing experience in the early stages of learning FreeCAD and Fusion 360. Unfortunately none of the CAD packages I've learned are good at explaining what the operator is doing wrong.
Lack of time, licence conditions, and being web-based have kept me away from Onshape. Although I prefer web-based F360 to SE, SE runs locally on my computer, and that outvotes F360s other advantages. The big problem with Fusion and other cloud products is the Vendor can change the Terms and Conditions at any time; for example Autodesk suddenly put a limit of ten open files on F360, which is frustrating when an Assembly is being worked on. The free version has been reduced in other ways, all intended to nudge freeloaders like me into buying a licence! The hobby version of SE isn't completely open either: it's licence time-expires after about 2.5 years, so I have to plan for that. But I'm safe for a few years!
Last week I thought I had nearly conquered SE, but it's bitten me with some new problems, including one where I realise my engine project has faulty foundations due to me not nailing details down early. The delay is keeping me from MOI, which looks powerful and easy to use. But, like the other free hobby CAD products, MOI's terms and conditions are a problem. There's also a limit to the number of different CAD packages I have the time and stamina to learn, no matter how wonderful a new one might be.
For all it's technical shortcomings, FreeCAD is the only Mechanical Engineering product I know of that's free as in speech AND free as in beer. It's licence is completely open, with no charges or restrictions. That's a considerable advantage, and it's good for developing single parts.
Dave