I've loaded for the one I had built in 1983, and four others.
There's no problems with the .257Robt. It's actually (with reloads) much better than the .243win that "killed" it off.
I've shot bullets from 75gr to 120gr. Only problem I've encountered was with a batch of Hornady 117gr BtSpt's. There was obviously two different lot#s of bullets in the box of 100. One lot# had very shiney jackets and a sharp edge around the base and a extruded demple on the base. These gave jacket seperations and were quickly sorted after I figured it out. However, they shot well to 100yds but destabilized and tumbled past 100yds. I didn't figure it out until I'd wasted 11 shots at 5 different mule deer on a Montana out-of-state hunt in '93. We kept thinking it was the shooter (me) or the rifle. After checking the rifle at 250yds and finding I couldn't hit a 3'x3' target, I switched to some 85gr Nosler Balistic Tips I'd loaded for prararie dogs. I got my muley on the next shot at 370yds. Day after, got over 40 prararie dogs out to 400+yds with remaining ammo... (85gr BalisticTips at 3,400fps over Win760).
Hornady did make good and replaced the box of bullets and sent me a coupon for additional discounts/rebates...
My favorite bullet "big-game" is the 117gr Sierra GameKing. Super bullet through either my Roberts or the .257wbymag. Always sub moa from every gun I've ever shot them through. However the 120gr HP GameKing (not a boattail) is also superb. They shoot 1-hole groups from both my Roberts and friends Interarms MkX in .25/06 (three shots. It'll do it repetitively, but not every time... sometimes the "wind" will get you!)
Another superlative bullet is the Hornady 100gr Spt. My tighest group ever with the .257robt was shot at 200yds over 48.5gr of H4831 (original surplus) and the 100gr Hornady at 3.000" (I have the "so-called" 3" chamber). It chrono's right at 3,000fps. Three shots touched.... but no-wind evening near sunset and no mirage...
Another "classic" Roberts load is 45.0gr of IMR4350 w/100gr bullet for 3,000fps. It shoots under MOA from my E.R.Shaw bbl and has taken over 100 whitetail deer including the three largest one's I've ever taken (over 200lbs dressed). My wife on oldest daughter too their first deer with the "Mr. Roberts" as my oldest daughter calls it.
The 100gr Hornady PtSpt is a "do all" bullet from the Roberts. It'll do anything the 115-120's will do. From my .257wbymag @ 3,700fps, it'll "stay" in a smallish deer at under 100yds. They'll be under the hide on the far side in a ~65gr "ball"... The "Interlok" feature works just as good as the much more expensive Nosler "Partitions". All three of the biggest deer I've killed were with the 100gr (2) or the 117gr Hornady BtSpt. Only one of the 117's have been recovered, and it weighed 109gr after penetrating rib, torso, and humerous of +200lb deer at ~240yds. Deer "broke" a 250lb scale I hung it from (field-dressed). I didn't think the deer was that big at first but several people commented on how big it was and encouraged me to weigh it.... It was "hanging" out of the bed of a Nissan pickup and sagging the springs.... It wasn't a "fat" deer, but very long, tall, and "rangey" as one taxidermist called it. Had an unremarkable 8pt rack and teeth suggested it was 7+yrs old.
But, I've never had a .25" Sierra shed a jacket, and with the better accuracy prefer them, however, for the past 4yrs, it's been hard to get Sierras.
It's a very rare Roberts that won't stabilize a conventional 117-120gr bullet. I've never seen one. However, I've heard of some early custom guns built from .250Savage bbls with a 1/12" twist that didn't like 110+gr bullets.
Only "issue" I've run into loading for the Roberts was a Browning BLR and A-bolt that have very, very, short throats. They required seating the bullets to SAAMI spec's to chamber properly. But, same powder charges worked nominally.
If you do get a Roberts and handload, do try IMR3031. It's a "sleeper" powder. At just 38.0gr it gives the "desired" 3,000fps with a 100gr bullet and excellent accuracy. It was the only truly MOA load from a Ruger M77MkII I had. I sold the Ruger for a $100 gain and bought an "orphaned" Ruger M77MkII "Hawkeye" "Alaksan" in .375Ruger. It's reliably a MOA gun (3-shots) and a thrill to shoot.... Especially enjoyable after I installed a Pachmayer Declerator "Old English" pad and moved the forward sling stud to the front face of the forend.
H4350 is my #2 powder.... Better velocity than IMR, but IMR is more accurate. YMMV