The comparisons between HRE and the Republic (Poland-Lithuania) are superficial. Probably the biggest difference is that Polish and Lithuanian magnates did not rule through titles (like "Duke of X", "Count of X", etc.) that encouraged consolidation of lands into quasi-state structures and then real state structures. The only such titles held by magnates were those awarded by foreign states for service, and domestic landed titles never arose because the fundamental idea of nobility was that all nobles, at least theoretically, are equal - landed or unlanded, wealthy or poor, they all, in principle, had the same rights and duties before the crown. The Imperial Diet represented titled landholders, but the Sejm represented all nobility, including the unlanded.
Granted, to an extent there was fragmentation. Magnates would go to war with each other within Republic territory and even independently waged foreign wars (the Polish invasion of Russia in support of False Dmitry in 1605, most famously). They often had private armies that overwhelmed central armies and used these armies to force their will, extract concessions, or refuse to pay taxes.
But I've always kind of liked the daimyo comparison more than the Holy Roman Empire comparison. Szlachta and samurai have some pretty interesting similarities, or at least szlachta is more alike samurai than any other European nobility (from their large number, to the vast number of impoverished nobles who essentially lived as peasants or as soldiers for their more powerful peers, to fragmentation that is more based on the hereditary lands of families rather than inheritance of titles...)