I have been researching and reading everything I can find on rollers and I have found a lot of discussion regarding the needle bearings not being hardened properly, not being virtually the same exact size and the lack of adequate oiling to the needle bearings and shafts. The needle bearings get hot. Traditionally they have depended on splash to oil them and this is often not adequate and particularly in an engine that is run hard.
Consequently, the major companies making rollers are putting oiling passages in the lifters to feed the needle bearings, shafts and rollers. In severe duty cases the needle bearings are not used at all. They install bronze bushings and also the oiling channels. I found a lot of broken link bars in retrofit stuff but it appears that the lifter failing has been the cause of the link bar failures. Once the link bar fails the lifter roller won't stay straight on the cam lobe and then you are likely looking at a catastrophic failure including fracturing the block and spewing needle bearings all down around the crank and rods. Below is a new style Isky bushed solid roller lifter with the bushing oil channel. The newer needle bearing rollers have a similar type oiling channel.
I found a tech article by a cam manufacturer that said they could detect the difference in friction between flat tappets and rollers on a spintron but on an engine dyno it was undetectable. Therefore, they are saying friction or lack of is not the reason rollers perform better. It is all in the cam lobe profile that makes it perform better. The down side of that has been that particularly with hydraulic rollers the extremely fast ramp speed has disrupted the lifter from pumping so you loose the top end rpms. Flat tappet builders went to anti-pump up lifters or semi-solid lifters to address this. Hydraulic rollers went to short travel to deal with it.
The question then becomes how much better is a roller cam. I found a 396 engine dyno test that did a flat tappet cam and a hydraulic roller of virtually identical grind and it got them 30 engine horsepower on a 396.
On my little 331 that might get 15 to 20 perhaps(just guessing based on my cubes and cam). I have to then determine if the added expense of a spider and bones kit, hydraulic roller lifters, new push rods and a hydraulic roller cam are worth the 20 flywheel horsepower. I can get a 244 @50, .576 lift hydraulic roller cam that is comparable to my solid flat tappet cam and it uses my same valve springs so I wouldn't have to pull the heads out to have them re-worked and new springs installed. They are already set up with new springs. My heads are limited presently to less than .600 lift. Is it even worth all the effort and cost to do a roller for less than .600 lift? Beats me.
So we get all the way back to the question of why the vehicle manufacturers moved to rollers to begin with. In a word, it was zinc. Technically, 3 words, lack of zinc. The EPA or whoever it was decided the zinc had to come out. Although technically the rollers don't have to have zinc, I found several reports that said they would benefit from it.
A myth I discovered is you don't have to break in a roller cam. This argument is used all the time for conversion to a roller cam and it is NOT TRUE. I found break in procedures for roller set ups including stressing the use of break in oils and the break in procedures are fairly similar to engines with flat tappet cams. The break in process is running in the whole engine and is concerned with a lot more than just the cam and lifters. However, even roller cams and lifters require a break in process according to the people that sell these things.
Roller lifters are also generally harder on the lifter bores and one cam manufacturer described solid rollers as valve jack hammers. Something in a higher lift solid roller with several hunded pound valve springs is seriously slamming the valves shut. Then you have to start looking at your valves and what kind of seats you need to run and how long will those last. This doesn't really sound like a good thing but the solid roller designers are working on all kinds of ways to lighten up the solid roller. Obviously, getting rid of the link bar is part of that because it adds significant weight to the thing.
I don't think the issues with roller set ups are the cams but I could be wrong. Every failure report I found was due to the roller lifter coming apart. So, I'm faced with the fundamental question, who makes decent hydraulic roller lifters and do I really want to convert.
Questions and comments are welcome. If I have read some misinformation in all of the reading I've been doing I will be glad to get better information.