It can seem really good on paper, but the problem is that Drizzle Swampert has a lot of things holding it back from being as great as it appears.
1. We have the other weather's, especially in the prominent ones, Desoland, Prim Sea, and Delta Stream used to mitigate weaknesses along with the occasional regular weather's too that would instantly end the regular rain that Swampert caused. This is one of the biggest problems for weather teams, not just Drizzle Mega-Swampert. This specifically is which Terrain teams, albeit a lot stricter in abilities needed/available, are often better than Weather Teams
2. Because Swampert will be mega-evolving, it cannot hold a Damp Rock to extend the turns of rain for 5-8, shortening his reign of terror. The mega stone also prevents him from holding a life orb or a choice band where other swift swim Pokemon like Volcanion can abuse specs in order to make it's special attack even scarier.
3. Once he mega evolves, you can no longer use drizzle as it's premega ability anymore, meaning that you need another rain setter to abuse Swift Swim again after those initial five turns are over, and generally the other rain setters are better than pre-mega swampert is in the first place. For a pre-mega ability, assuming you do have another rain setter to continually abuse swift swim, Download is a better use and burn ability, giving it +1 attack sometimes, depending on what you send it in on.
4. In a metagame where speed abilities can be used a lot more, outspeeding +2 base 70 isn't all that hard, especially when +1 base 110 already does it (524 for +2 base 70, and 525 for +1 base 110), meaning that it's revenge killing it on speed alone can be significantly easier than in other metagames.
In the end, even though Drizzle Mega Swampert seems cool, there are simply a lot of things holding it back that one should be warned of before using it.
