First, they are very wobbly and you're gonna be limited by that. You really need a fair amount of surface contact and a robust counterweight when you're extending the arm and moving boulders, pulling out roots, or digging into hard soil.
Second, you might look at specs and say "oh, cool, digging depth of 5 ft, I don't need more!". Except, this is attainable in one specific (and awkward) position of the excavator's boom and arm. It's less than that across most of the movement range.
Another thing to keep in mind that this is not like buying a car. An excavator or a tractor deprecates far more slowly, and many private owners keep them for 20-30 years. A diesel excavator will need DIY maintenance and simple repairs, but it should hold up for life. Battery-powered equipment will probably need new battery packs before that. Just something to keep in mind when figuring out the pros and cons. If you're in the suburbs, noise is a concern, but then, you honestly don't need your own excavator...