There are two sorts of imbalance: bananas and wedges.
Bananas are logs that are basically circular, but curved. They can be seriously out of balance, and they're unpredictable. It takes a very practiced eye to see just what's obtainable from inside such a log, so the easiest way is often to mount it up in the lathe and take a quick skim. Very often you'll re-adjust the centre position, as you see how your best cylinder actually fits inside it. For sawing it lengthways to remove the lumps, the lathe is actually a better tool - especially as it's pretty much self-guiding for where to cut. If you do cut, it's usually to chop the length. Two shorter logs, of equal curvature radius, give a more useful middle.
Wedges are made by splitting a big log into quarters. This also avoids the central pith, which is often a good idea for stability. Historically such a billet would then be held in a shave horse and shaved round with a drawknife (traditional work for a bodger) before being turned in a semi-rotary pole lathe. As a drawknife along the grain removes wood with less effort than a pole lathe and gouge across the grain, that's worth the effort. With a motorised lathe, I can just be lazy. The off-balance from a wedge billet is generally small and predictable, so it's no big deal.
Then there are rough bowl blanks. The size of the biggest bowl you can turn is generally limited by your fear level when first roughing it. This is why smaller, but massively constructed, cast iron lathes of the '50s & '60s like Unions and Graduates are still so popular, especially for bowls.
It has two good features: a solid cast iron bed, also the variable speed drive. These are well worth the increment from £100 to £200. You also get a floor stand, which you'd have to provide somehow anyway.