Given that they built only nineteen of the QF6 pounder (57mm) cannon Mosquitos, it clearly was not perceived as a success. They preferred the 60lb rockets.
As I noted in an earlier post, the 17 (not 19) Mk.XVIII were built in small batches of 2 or 3 airframes at a time, between Sept 1943 and Jan 1945. And they remained in squadron service from Oct 1943 until the end of the war. The modifications to the aircraft were extensive to withstand the shock of the gun's recoil. A lot of armour plate was added around the nose and engines, adding to aircraft weight and affecting performance. So they were very much a niche aircraft.
And it reduced the flexibility of the Mosquito. The gun took up so much space in the belly that there was no space for bombs or fuel aft of the gun as in a normal FB.VI. They always seem to have flown with 50 gal drop tanks on the wings, so no bomb / rocket carriage there.
In squadron service they had to be flown by specially trained crews, and the attack profiles called for very accurate flying to get the rounds on target. But on each attack run it was generally only possible to get about 5 rounds off on each firing pass, such was the closing speed on the target. Tse Tse were generally accompanied by other flak suppression aircraft while they made their runs. So the weapon worked well against isolated U-boats fighting it out on the surface in the Bay of Biscay where multiple attack runs could be made. That was what it was originally built for in 1943.
It didn't work so well in the kind of strikes practiced by the strike wings 1943-45 where the objective was to swamp a target, usually a convoy with multiple escorts, in a single.pass lasting 2-4 minutes, although they were used in such attacks. It is worthy of note that when withdrawn from 248 squadron at Banff in March 1945, the whole Tse Tse Flight was transferred to 254 squadron at North Coates specifically to target midget submarines being encountered along the Dutch coast.
The 3" rockets and their associated rails could be fitted to any FB.VI fitted with the appropriate wing strengthening strips on the production line I.e. most of them by 1944. 3" rockets didn't reach the Coastal Command Mosquito squadrons until after they moved to Banff in Sept 1944 (first rocket operations in Oct/Nov IIRC). And the initial fit precluded the use of drop tanks (4 rails on each wing) so limiting their range and ability to roam along the Norwegian coast. It wasn't until Feb 1945 that the 100 gal drop tank with 2 double tiered rocket rails on each wing was fitted after the Wing was withdrawn from action for a week or so to have the refit work carried out on the station by the squadron ground crews. The first operation in that configuration was flown on 7 March 1945.
So for more than half it's operational life the Tse Tse was not in competition with rocket equipped Mossies. And for another 6 months they operated alongside each other. So I don't think it was a perception that they were not a success. Given the small numbers in service at any one time (usually no more than approx 4) it would have been easy to drop them from service. But they didn't.
Edit:- trials with the FB.VI with rockets didn't take place until the end of Sept 1944. They proved successful and were quickly fitted to the Banff Wing Mossies who flew their first rocket equipped mission on 26 Oct 1944 after training on the local bombing ranges. Prior to that the call in Coastal Command had been for more Tse Tse.