Yes. Even with points but in the wrong places, 5D can be poor.
True, but that is why it is best to have a way out. In this case, you can always outbid partner in diamonds if s/he keeps bidding clubs. Note that 3C does not have to show a genuine suit, so that should help partner not get too excited. If you open 1D and have a hand that us too strong to rebid 3D (because that is non-forcing), you may manufacture a 3C bid to get the game force established. Say you hold x KJx AKQxxx AQx. You open 1D and partner bids 1H. Now what? You are too strong to bid 3D, plus partner might pass when 4H is good (xxxx AQxxx xx xx). You cannot bid 3NT with a singleton spade. The solution is to bid 3C. If partner bids 3D, you can bid 3H to show your 3-card support and, by implication, short spades.
I once cue bid 4D, which got doubled. Partner passed it back to me and I redoubled to show that I had first round control (our cue bids only show first or second round control). I had a void. Partner passed! RHO then bid something to save us playing 4D redoubled on a 4-0 fit.