Where do you live? There are some good sources of pre-fab in California.

Pre-fabs are great if you want a relatively popular type of granite and have typical cabinet set ups and sizes.

Prefab usually comes in blanks that are 25"x72". In some coastal locations where freight is cheap, you can find the longer blanks, say 108". Here, the discount building material stores import containers of them straight from China and the builders snap them up like crazy. The builders usually have a worker cut them to size and seam them and do a top mount sink cutout. Lots of seams usually result from using blanks. Lots of BAD seams. Chipped edges and non matching epoxy. Actual granite fabricators here won't touch them, so you never get a good quality product. DIYers also try to work with them, and if the counters are short enough, and they are careful enough, you can get a passable result.

In other locations, I hear that you can get actual people experienced with working with stone to do some minimal fabricating for you with them. I'd still be mightily suspicious, because any good fabricator wants to be able to give you good results, and using blanks just isn't the product for top notch results. It IS cheap. But that word has two definitions.

Pre-fab granite can be fine. They will need a seam at any corner since they are straight runs. That may be what live wire oak means by needing lots of seams. If you have straight runs, then you will only need seams for runs that are longer than the available pre-fab blanks.

The pre-fabs that I've seen generally are 2 cm in counter lengths with the edge already laminated and polished. The fabricator has to cut them to length and make any cutouts for things like sinks and cooktops. They are usually more uniform stones.

When we needed a small piece of uniform black granite for our hearth (about 2' by 4'0), we found it in a Shanxi black prefab blank. This one was marked "island" and was 3' by 8' instead of the usual counter depth - I don't know how someone is suppose to make an island out of that unless they use two and seam all the way across or it's a narrow island. It had a polished edge on one short and one long side. It cost us $10 per square foot.

There was certainly nothing wrong with that granite and our fabricator made no objection to using it. It is a lovely uniform black and our fabricator said that it was even harder than our quartzite. And he was a good fabricator - he made nice tight clean seams on the fireplace surround which had to be done in 3 pieces because we were using the remnants from our kitchen counters.