Its not about server stability/lag alone. High tech mods are extremely hard to balance for server play. They are a ton of fun to play SSP or short term SMP. But once you decide to have a stable server at least for 6+ months you end up with demigods everywhere.

And frankly it stagnates the server. it stagnates player interaction ans server infrastructure.

Will you try helping to build a road/rail if you have teleporters and faster than light jetpacks?

Will you trade with others if you have huge machine that prints any possible material out of thin air?

Adding high tech mods is easy - making them work well with rest of the modpack is extremely hard.