veganbikes

No issues with linear pull brakes which are what a Shimano trademarked "V-Brake" is. Also no issues really with cantilever brakes but I do hate setting those up. I haven't really had any time with center pull stuff as that was more popular in the 70s and before and is rarely seen past the 80s except on some Neo-retro type builds or actual vintage stuff.

A bike made from 30 packs of underwear and a platoon sized jar of peanut butter filled pretzels is going to have crap brakes, well everything crap. Costco sells bulk items at low prices they are a warehouse filled with all sorts of junk. You can sometimes get some excellent deals but a place to buy bikes it is not. They are not a bike shop, they don't have mechanics on hand and the stuff they sell is usually either similar to wally mart or maybe a minor notch up but barely.

I personally wouldn't put any money into the bike and ride it into the ground and buy something with better everything. However as some have said you could get a set of cheaper Shimano or Tektro linear pull brakes and that would be a massive upgrade.

When braking you don't want a lot of flex so if you have flexing arms and rubber molded pads that don't really have a stiff shoe you will have poor braking. Also add in cheap levers with a lot of play and that will just turn into poor braking.
If your arms and levers were decent (which they aren't in this specific case) then I would say get a good set of shoes and pads to put in them from Kool Stop or SwissStop, no once piece stuff two separate pieces. I would also consider getting good polished non-coated stainless steel cables and a good compressionless housing like Jagwire Pro or Elite link kits (they come with everything you will need generally and are a good upgrade) and you can have much better braking without spending a ton. Pads and shoes are often overlooked and people just say give me the cheap stuff but for an extra $5-10 if that you can have really good stuff and improve braking significantly I don't see why you wouldn't.