The problem is not that clump-formers can run, in general they are botanically incapable of it (Yushania being the noted exception). The true problems are
(1) misuse of the term "clump-forming" by most people who sell bamboo, especially in the case of phyllostachys
(2) false expectations of people who somehow hope that bamboo won't transgress outside the one-foot circle they put it in - they fail to understand how big the bamboo will grow and what the realistic size of the clump is.
The true clumpers that are sold in this country and really do clump are basically Fargesia, Thamnocalamus and Borinda. Not that some of the foregoing are not quite big. Some of the smaller Fargesias really are going to remain within a 3 foot circle, especially Fargesia rufa (well that's what it's commonly sold as, it has actually been renamed botanically).

The situation with Phyllostachys is that the most popular two kinds, P nigra (true nigra, not other varieties) nigra and P aurea (most varieties) generally do stay put in British conditions, within about a 6 foot circle anyway - though that would be considered transgressive by those who hope for miracles. They are nonetheless invasive if conditions are right, ie warm and damp enough. People in parts of the USA where these things grow well correctly think of them as invasive. Someone from North Kent was showing pictures on another forum of his P nigra being invasive, but that is fairly unusual unless you live in Cornwall. So garden centres are not distorting things too much to describe those ones as being in practice mostly clump forming in British conditions. But the problem is when they remove the caveats, and then apply the description to other Phyllostachys which are a bit more inquisitive at the root, like P nigra boryana, or P aureosulcata.

The Phyllostachys are lovely, and if one likes them and has realistic expectations, and puts a rhizome barrier along ones property boundary just to make sure if you are growing them near the neighbour, they are not too much of a problem to grow if you like them. They provide a year round barrier, unlike most other grasses.

Most bamboos really don't like it in a pot. You can grow them. If you grow a phyllostachys, you'll have to divide and repot it every second year. And they need a lot of water, and you would worry about them dying while you go on holiday. In practice some of the smaller Fargesias perform best in a pot, but you may as well put them in the ground.

To get further detailed info on bamboos, and discover how many lovely Fargesias there are (among others), I suggest browsing the websites of Panglobal plants, Jungle giants and uk-bamboos.co.uk (Whitelea). Jungle giants is more prone to misuse "clumpforming" than the others, but he has little drawings to show you what kind of effect the bamboo is likely to have, and has the best mail order service. Panglobal plants has one of the very best selections of bamboos, but no mail order and no pictures.

The problem with other grasses as a screen is that most of the taller ones die back in winter. But there are notable exceptions. If you want an evergreen larger grass that will do pretty well in a pot, consider the restios, which are a South African group of reed-like grasses which are sort of half way to bamboo. Trevena Cross is a major mail order seller of restios, and has a good website. I am growing Thamnochortus insignis in the ground, and I haven't had any problems with frost, although some new shoots that foolishly came up in mild Feb got killed off by April frosts.