onrush:
In the past I've used tongue-and-grooved flanges for many years - primarily in ammonia refrigeration service. The main attribute of using these type of flanges (although not mentioned yet) is that they inherently prevent a gasket "blow-out". I have never come across, seen, or heard of any shortcomings regarding mounting and machining. In fact, I've never had a leaky tongue-and-groove flange - and that is quite an achievement in ammonia service.
The obvious trade-offs include price and weight. I always used Vogt's design (found in their old F-11 catalog) and I've applied the type up to 3,000 psig working pressure. I always found them available and in stock - like other conventional raised face flanges when I bought them from Vogt. The material of construction I've used is forged steel. I would not hesitate using them if they are from a recognized supply - except that the price and the weight will be higher. I would certainly prefer them over RF type in ammonia service - based on my experience.
Like ring joints and lap joints, I believe they have had their "niche" in certain applications.
To quote the Navco Piping Datalog:
"The small tongue and goove type of face was developed originally for hydraulic service to effect a high unit gasket pressure in a confined area. It has the disadvantages of excessive gasket pressure under temperature change and pipe deflection, mushrooming of tongue under excessive bolt loading, and requires special attention in machining and assembly. The large tongue and groove type face has the merit of gasket retention with lower unit gasket pressure than the small tongue and goove. The gasket area is nearer the bolt circle making it slightly more resisitant to flange stresses and bending moments. Special care must be exercised in machining and assembly."
My experience concurs with Navco's opinion.