I am not arguing that the Ross is a better battle rifle then the Lee Enfield. What I am trying to say is it had been tailored towards a different type of combat (long range battlefields where accuracy is everything) then trench warfare due to the mindset of the time (WWI was a steep learning curve for everyone). I don't think the Mk. 3 Ross design could be truly effective in the type of conditions required for trench warfare just due to the intricacy of the design. However there are battlefields where the design would shine.
The complexity issue is pretty much a non-starter IMO, except insofar as the mag is concerned, though the SMLE is easier to clean in some respects such as the bolt which is also easier to remove and replace. Overall I'd say a Ross MkIII with a barrel of SMLE length and a nose cap adapted to the 1907 SMLE bayonet would have been very comparable as a combat rifle. With it's much heavier barrel it was not only much more accurate but more suitable for prolonged firing. The Ross had better sights by far, but the balance and heft was definitely inferior to the SMLE.
Quick summation...
Ross Defects:
Non-detachable 5 shot magazine vulnerable to denting vs. the ribbed, detachable 10 round magazine of the SMLE.
Barrel too long.
Bayonet too short and inferior to Patt. 1907 for thrusting and intimidation(!)
Stock wrist too thick.
Buttplate steel, not brass (prone to rust)
Barrel not fully enclosed by upper handguard (ingress of dirt and moisture under the barrel and potential for burns to the hand from heated barrel)
One piece stock harder to adapt to soldiers length of pull; butt cannot be changed separately (also less convenient for maintenance).
Cannot be manually re-cocked without opening bolt.
No provision for half-####.
Safety cannot be operated without removing hand from stock.
SMLE Defects:
Open sights inferior to aperture sights
Barrel too light for best accuracy or sustained fire, vulnerable to distortion of forend affecting accuracy.
Trigger mounted on guard vulnerable to distortion of forend affecting function and harder to manufacture and maintain.
Stock & handguards more complex and expensive to manufacture and fit.
It's funny how many reported complaints there were about the the length of the Ross and how you couldn't get down into dugouts with it slung, hitting the bracing over trenches etc and yet from the photos it looks like a lot of men kept their bayonets on their SMLEs almost permanently in the trenches, making them almost as long as the Ross had been with its shorter bayonet fitted. Simply psychology!