Modern keyed brass plays a lot better than the originals. You can now buy instruments that actually play in tune.
They have different properties than valved brass, just as various types of valved brass have different properties from one another. Each type is its own experience- Vienna valves, Stolzel valves, Perinet, rotary, Berliner.
I don't like the sound of the modern Bb trumpet. Never have. It sounds like it is- a type of cornet. The sound has no life, no character. Even a good cornet sounds better to my ears than a Perinet Bb trumpet. It especially doesn't cut the mouse-turd where 19th century and earlier music is concerned. The sound doesn't roll. It can't sound Gothic, or martial, or stark, or bleak, or much of anything. It's a kind of blah one-trick pony that just does what it does.
Modern orchestral players especially are fixated on technique and tone-production at the expense of performance. Great tone, great technique, extremely boring performances, with the conductors milking dust out of the old cow's teats.
The main difference between a cornet (which the modern Bb Perinet "trumpet" is) and a real trumpet is this:
The cornet is and always has been a circus instrument. "Look at me! Look at me! Listen how high I can play! Listen how fast I can tongue! Okay, I'm going to play you a ballad, now. How about another ballad? And another? Watch me do my nips and flips, now! Watch me at centre stage as I play Paganini's Moto Perpetuo 83 times at super-duper high-speed in one breath as I jump from a platform into a bucket of boiling fat."
That is not and never has been the trumpet's shtick. Real trumpets are imperfect, with rough edges, they're hard to manage, they're not your best friend in a tight spot, and won't let you lay back in your comfort zone. But unlike the cornet, they have something meaningful to say. The trumpet is not simply an entertainer's instrument.