Most consumer level hand hedge shears are blunt and need one blade
sharpened at some angle of 30 degrees or less.
Keeping a straight line requires using the tool properly which is
rarely ever seen. You keep one arm straight and open and close the
shears with the other.
Operating them like you are trying to clap your knuckles together
makes for a poor job.
There is no shame in dragging out a string to set a reference , nor is
there any shame in going down the line with a pair of lopping shears
to remove heavier wood first.
Properly done you should not see any woody stubs on the surface you
have trimmed, these should all be back a few inches.
For around 100 bucks you can buy a pair of ARS k-1000 or k- 1000l
which are a joy to use.
There are knock off's which are not too bad as well ,Lee Valley
EC-520 $35.50

For less money you can still find good quality Wallace, Wiss, or
Wilkenson shears in yard sales and flea markets and tune them up or
have your local sharpening shop do the work.

master gardener, 25 years full time sharpener, beekeeper, curmudgeon