V-drive transmission in a 31 silverton with 454's what exactly is a V-drive transmission? good or bad?

V drive allows the engine to be mounted further aft in the boat, backwards - flywheel forward. Good for more room in the cabin, and puts the engines under the rear deck where access to them is probably better.

Aside from that, they can be noisy & expensive to repair. Depending on the hull the weight aft might not be so great.

So for long life and use stay away from them
They were in a 1999 Silverton with 454 's
I was comparing that to a 2000 wellcraft with 5.7's volvo penta's maybe I'll go with the wellcraft

nobody said to stay away from them.

They can be expensive to repair IF they need repair.
Regular fluid changes will help them just like anything else.

I had several Warner V-drives and they proved durable and reliable. One got a cooler cclogged up and it overheated until it began to slip, but I fixed the cooler - changed the fluid and sold it several hundred hours later with no more hiccups.

I like em.

I got some big Hurth V-drives in the Sea Ray. Manual says they will take "one" full power reversal at speed, but to only consider such a maneuver in a situation where life or limb is in peril. Sounds kind of fun in a warped sort of way.