I've conducted extensive testing and currently run a UAD Apollo, Antelope Audio, and Apogee Symphony MKII all at once. Honestly, using three devices simultaneously only becomes a challenge if I'm tracking a LOT of channels at the same time. This setup also includes a Behringer ADA 8200 connected via ADAT.
In the past, I used a master clock with a Lynx system and Big Ben, but eventually realized it was overkill. Clocking technology these days is fantastic compared to the 2000s. The internal clock on your main device is usually more than enough, and in most situations, it's actually better to use internal clocking.
However, Antelope excels in clock technology (though other aspects of their products might not be as impressive). They utilize a "oven" technology that controls the crystal temperature, a key factor in preventing clock drift. RME makes outstanding products, so don't fix a problem that doesn't exist.