Learn Italian: I nuovi sistemi non sono migliori, sono peggiori. - The new systems are not better, they are worse.

Sistema is a masculine noun, even though it ends in -a in the singular. It belongs to a group of Greek-origin words ending in -ma (like il problema and il programma) that are masculine. In the plural, the ending changes to -i (sistemi) and it takes masculine plural articles and adjectives like I nuovi.

Why do migliori and peggiori end in -i here?

They are describing the plural noun sistemi. The singular forms of these adjectives are migliore (better) and peggiore (worse). Since they end in -e in the singular, they change to -i for both masculine and feminine plural nouns.

Could I say più buoni and più cattivi instead of migliori and peggiori?

You will sometimes hear those regular forms, but migliori and peggiori are the standard, much more common irregular comparatives for better and worse when comparing the overall quality of things like systems.

Why is the word 'they' missing before the second sono?