I've had a tank of rock flower anemones for about 10 months now, and they're great. Easy, colorful, peaceful and mostly unmoving once settled. That being said, Clown fish will not host them, and shouldn't, because they aren't species that live in the same ocean together.
I keep rock flowers in my heavily coral-stocked tank and I'm not at all worried about them. I also put the rock flowers in after the tank was only a few months old.
Now on the other hand, I have a relatively newly acquired and rather large black widow BTA, and the tank it went into was 2 yrs old and had very few corals. I knew I was more or less giving the top of the tank to the anemone, and I'm going to drop some easy, colorful things like zoas and acans on the lower levels. Things I can either move, or I wouldn't be too annoyed with losing.
As for the being sucked into power head part - I never want that to happen, so I purchased a nem guard for my powerhead. If it ever detaches to float around, the worst that could happen is it gets stuck against the outside - unharmed. I do have to keep the guard clean or it'll stifle flow, but that's a small price to pay for peace of mind.
This video a couple of weeks after It settled does it no justice, and I ended up moving those zoas down to the bottom rocks. Some of them stayed since they'd grown off the plug, and they're strangely doing fine.
Ultimately it depends on what you want to see out of your tank