Should 9.75" radius curve pieces be outlawed?

Another novice layout mistake - using 9.75" radius curves.  I knew they could be "difficult" so I  limited the number I used on the layout.  My first two locos were a Kato C30-7 Santa Fe (love this loco) and  Bachmann EMD DD40AX.  The DD40 was a great price and came with DCC included.  What my novice N Scale self didn't realize is this thing was a monster - nearly a foot long!!!!  My next loco was a Kato SD40-2 BNSF, which was also sweet looking.  Here's a comparison of the three locos side-by-side:

I sold the Bachmann on Ebay and broke even - no loss, so that's good.  It was a good loco but not for my layout.  Now, back to the dreaded 9.75" radius curves.....

The reversing loop, which will be hidden under the upper deck, was about half 11" radius and half 9.75" radius.  The Santa Fe C30 tracked around it with no problems.  The SD40 - it was another issue.  That bad boy WOULD NOT stop derailing on that loop.  I knew that something had to happen so I pulled up the loop and re-installed it, with a little easier curve.  Again - SD40 derailed.  I rebuilt it a total of three times before I finally was able to get the SD40 around, in forward & reverse, at full speed.  I know the locos will never run at full speed but I had to be sure that things would track (pun intended) once the upper deck "lid" was in place.  Here's a view of the loop in question being reworked:

Problem reversing loop with Sheila's school bus and backhoe shown.

In follow up research I learned that 3X a car length should be the minimum radius used & 5X is best.  Now it makes sense, 40' boxcars are 3.25" long.  Use the 3X factor and yu end up with the 9.75" radius.

I know that the 9.75" radius pieces have a place - but in the future, it that place won't be on my layouts!