The one piece seal uses a crankshaft designed for the seal. You cannot use an older crank in a newer block. You can however use the newer crank in an older block with the GM one piece rear main seal modification kit. The one piece seal is a vast improvement in an area the small block Chevy has needed ever since it's inception. Rear main seal leaks are almost a thing of the past in the newer small blocks.
I don't know who told you they were less desirable than an older block, but IMO they do not know what they are talking about. The only way I would use an older block would be in a period correct restoration.