My responses have been in view of the OP and the subsequent responses. A modern Shiloh Sharps will function with properly loaded smokeless rounds and a improperly loaded smokeless round will render pieces of said rifle but that is true for any modern firearm. I agree that a blanket statement in regards to hundred plus year old firearms should be loaded with caution the same concerns can also apply to black powder as it also can do harm. For years the use of black powder has engrained practices that when applied to loading smokeless cause great harm to gun and shooter. That is no reason to not use the powder as too many people practice safe methods of loading rounds and fire them pregularly in complete safety.
I have fired many a round of black powder and choose today to not use the powder, the smokeless is much easier to load, imo. I also will advise any novice to stay away from smokeless, for that matter black powder until they understand the dangers of each. 
My close friend bought a new 50/90 from Shiloh years past and against their advice has never fired one round of black powder in the rifle, choosing to use years of experience to develop a smokeless load. Judging from his loads and the usage by numerous others with their Fifties I feel comfortable with my earlier posts.
My past experiences lead me to believe the real problem is the folks that look at smokeless with black powder "eyes" and then point fingers at those that use their methods to explode their prized single shots with the evil nitro.
Matches are won and lost all the time with straight wall cases using the correct amounts of smokeless and there is no reason the 50/90 can't be loaded in the same manner, to think otherwise borders on irrational.
That said, I don't load smokeless in nothing but modern American and Japanese copies of Winchester High Walls, I don't like narrow margins of safety!

I have to agree with Harry on the use of BP in BPCR..as well as the old time 10# max. weight & 3# min. single trigger rule for long-range match shooting.  If we are truly a time bound (1865 thru first WW period) orginization.  Years ago, we invited some BP silhouette shooters to our clubs schuetzen match.  I felt ashamed at the way these new guys were snubbed by the regular members for their use of BP..could hear wispered snide remarks and curses behind shooting line during the match.  What hiprocracy!  I wonder if these same fellows would have derided Mr. Pope shooting at one of the huge schuetzen fests for using black in his muzzleloading rifles?  Creedmoor type long-range,as well as schuetzen matches all began with black powder!...and the earliest rifles used in both were muzzleloaders.  To discriminate against a competetor for using a traditional propellant because of...my heavens..a little smoke is indeed a sad state of affairs in the shooting sports.

I don't have dog in the fight of one powder over another, I do feel there is room to grow when dealing with the two powders, especially in the smokeless area where a vast amount of differences occur in it's use.
   I shoot mostly club matches for one reason, they are not usually divisive in the rules concerning powder. The line is usually equally divided with shooters of both powders and the smokeless shooters complaining about the smoke for sure but in a light hearted manner, those that think otherwise can go their merry way so to speak. I have found that most black powders like to shoot next to a group of smokeless shooters if possible:-).
   I do feel that both types of powder can be used a events if the two sides choose to work at it and if the study of safety is done correctly the naysayers would be few and far between, our sport would grow rather than stagnate. 
   The mention of Harry Pope brings to mind a person that was a shooter rather than a purist and if given his choice of powder types I dare say he would choose the one that gave him the best results at the target.
    The OP asked a question that will be asked a thousand more times and a negative response plants a seed that will not help the sport. The use of modern steels meet the demands of any powder on the market today and knowledge should be shared to those that need it, to not is to be unsafe and unwise at the same time.

Well while you guys are helping sorting a few things out here I've been busy getting my 50/90 reloading tools and supplies in place. I even got my cases annealed with our Series II case annealing tool. 

Thanks to HiWall55 and Lee for sending me some cast bullets to test. I'm still looking for a quality mold for 600-700 grain long range bullets!

More to follow. Thanks everyone! 

High Noon

Blackhorn 209 has data for loading 50-90.  I have used BH in 45-70, 38-55 and 30-30 with excellent results. It does require a magnum primer for uniform ignition, but this is not an issue.  I like the fact that it fills the case and seems to work best with a bit of compression.  Velocities are very uniform.  I've shot out to 500 yards with excellent accuracy in with 45-70 and 38-55 loads. It gives the full case advantage of BP with easy of use with SP. Only draw back is that it is expensive. 
Mike.