I must be the only person on this forum that actually LIKES the Northstar lol

What is the main problem with them?

Head studs. The method GM used form the factory fails anywhere from 50k up (mine is 128k and no issues yet). The studs start pulling the sleeve up which causes the entire head to lift off the deck (obviously nm in amount, so not visually noticeable) which leads to gasket failure and coolant leaking into exhaust. I plan on rebuilding mine later this year or early next. $289 for the permanent fix, which permanently mounts the studs to the block with large coarse threads. One time job and done. The Northstar sounds amazing (especially with my Flowmaster 40s) and is fun to drive for being a FWD car. Not saying the 3800 motor is bad (I might be getting one for a project that is in the works) but the Bonneville deserves a V8.

it should have been a ls engine.

Yes and no. While the LS4 would have been nice, the Bonneville was a luxury car with a sport attitude so putting a Cadillac motor in it made sense. It is the same car as a Seville after all

i wouldnt buy any kind of caddy. or anything with a caddy engine they are always in the shop. they always have issues with other electrical things, all sorts of stuff not just the one issue.

the ls engines go 2 to 300k like the 3800'* and you can mod them as much as you like. the ones in the fbodies and corvettes get mid twenties for fuel economy too.

Not sure there is a permenant fix for the N*? The first head gasket went at 122K miles as anticipated failure, but one year and 30K easy miles later with studded heads by "claimed" N* expert the second head gasket failed. Personal opinion is find a different auto, one that does not have a N*. Got nearly 300K out of my 3.8 in a 95 Olds without any major work to engine. LOL

Still in process diagnosing the cause of second HG failure.

most of the regular 3800'* will go 300 no problem, and my inlaws got over 30mpg the whole time till the body started rusting enough the frame cracked. mine only gets 28 but its got different gears and my wife drive it like john force

Depends on which "claimed Northstar" expert you used. I know of one who has a pretty crappy product...

Product??, the basic problems are the skills of the mechanic or lack thereof.

Back in the days when I owned a Rover 3500S and was in the parts department
of the dealer, a mechanic came over, made small talk then made the statement
"they're junk cars". I thought about that a second and replied they're no better then
the mechanic working on them!"

Since I was a teen I've worked on all my cars as a hobby, never did any need major engine work and I get 200K to 300K mi per vehicle without any major work. Give it to your typical shop mechanic and they'll turn it in "JUNK" the first time they work on it.

After the second go around on the N* HG issue I decided to pull the valve cover on the front head. The studs were misplaced in the block! The top left and top right nuts were way, way over torqued and the remainder 8 nuts were randomly torqued between 55 to 75 ft/lbs when all should have been torqued to 75ft/lbs.

Judge for yourself very complex engine maybe way over the heads (no pun intended) of your basic mechanic but they should at least be able to do the very basic of engine tasks, i.e. torque head studs!