Inaccurate blanket statement, unfortunately.
There are times when it absolutely makes sense to redeem for Y instead of selling and paying for Y.
@OP, you're going to have to provide some info:
1) Do you want to fly First? Business? Coach?
2) When?
3) Stopovers?
Also, IIRC, the only airline that both UR and MR transfer to is BA, but redeeming for long-haul flight on BA has YQ.
From NYC to TLV, the lowest it gets is 80k amex/chase which assuming a 1.5 cents per mile value is $1200. You don't earn miles from the flight, so that's $100 lost.
So the only way it is viable is if u can't find a revenue ticket for less than $1300, and that's assiming u can find award availabiliy on UA w/o taxes. If you can't, that's $1400 or so.
I doubt between now & EOS there's a day where u can't get a flight for $1400, but can get an award with less than 2 stopovers.
The exception is if u fly EI NYC-DUB which is only 27k Amex RT (or BOS-DUB for 17k amex). Only there are virtually no flights DUB-TLV, so u need easyjet to BSL or so, which is about $400 RT DUB-BSL-TLV.
27k * 1.5 makes $405 + about $100 BA fees $505 + $400 for easyjet - ~$900 total.