you will find that a gadget that works for carrots does not work so well for onions, but one that does well on onions doesn't work well on sweet peppers, but the one that handles peppers can't do potatoes worth a hoot . . .
a chef's knife is your friend. a dull knife is your worst enemy.
learn how to sharpen, and keep sharp, you knives. life turns out so much easier....
slicing and dicing with a dull knife simply does not go well.
for hard(er) stuff, first slice it into long thin strips, then dice the strips into little pieces. carrots, parsnips, leeks, celery . . .
for round things, cut them in half, then make slices, cut the wide slices into strips, then dice the strips.
rutabaga, beets, celeriac, turnips . . .
onions - a special case - thousands of videos showing (1) cut in half (2) horizontal slices (3) vertical slices (4) slice off small dices. the size of the dice depends on how many horizontal and vertical slices you make on the "half onion"
if you are nervous about the horizontal slicing, there's another method - cut into quarters, do the vertical slices, rotate the quarter 90', do more vertical slices, then slice off the dices.....
the most versatile, altho still limited,,,, gadget is probably the "slap chopper"
these have been around for tens of decades - near a century perhaps....
if you search on slap chopper you'll get some very amusing 'as seen on TV' videos.
guys with dull knifes cutting up an onion, leaving the root, and trying to make little pieces of it.....