At our start-up we have a history of deeply understanding our users' needs and can create loyalty and retention. But we find we are so careful about getting it "right" when it comes to what our product does that we lose the succint, repeatable marketing message. Do you feel that a succinct, hook-worthy statement that is 80% "right" is better than a "right" statement that is convoluted and hard to remember?

I love this question because balancing what feels like competing goals is one of the most challenging aspects of being a product marketer. The reality is that these aren't competing but complementary goals, and it's our job to do both -- deeply understand our customers' needs AND develop clear, compelling messaging. While we don't have the luxury of choosing between 80% "right" messaging or 100% "right" messaging, the good news is that 100% right is an unrealistic goal, and there are many incremental improvements you can make right away to improve on 80%. Here are some ways I've tackled similar challenges

• Develop separate, distinct personas - You mentioned your existing users, but are you addressing the needs of new users or buyers? It's possible that the messaging you consider 80% for existing users is 100% for new users and vice versa. Because each persona's needs often vary wildly, create separate personas to guide messaging that speaks to each. 
• Tailor messaging for each persona - Effective messaging resonates with a specific audience, and it's unlikely that one-size-fits-all messaging will work well for all audiences. The easiest way to get from 80% to 90% is to tailor your messaging specifically to the needs of each persona.
• Test, improve, repeat - Lean into both quantitative and qualitative testing. A/B test landing pages, emails, and ads. Conduct user testing and share variations of the messaging with a good sample size for each persona. Ask internal stakeholders (product, sales, customer service, etc.) for feedback and partner with them to quickly test messaging "in the wild" with prospects and customers. 
• Seek inspiration - It's comforting to keep in mind that someone else has probably faced the same challenge, so I look to other companies for inspiration. Ask yourself which other products translate complex needs or complex offerings into concise, compelling messaging -- bonus points if they also speak to multiple personas. A few that come to mind for me are Descript , Drift , Zendesk. 
• Keep it simple - Complex customer needs and complex products do not require complex messaging. Messaging should simply convey why a customer should care. Granular details can be captured in FAQs, documentation, and supporting product materials. When in doubt, leave it out.

A few thoughts on this one...

I would focus less on whether your message is a certain length or tone but is the messaging serving the purpose as it aligns to your business objectives. I talked a bit about how to measure whether your messaging is working a bit earlier so take a look at that answer to determine how to measure the effectiveness of your messaging. 

From a messaging perspective, we always strive for brevity eventhought it's challenging...how can we slim down what we're trying to say but still get our point across. 

In order for messaging to be effective, I believe it needs to be effective. If not everyone is saying the same thing when asked what your company does and how it's different then your messaging needs to be updated. 

Also, make sure you think about where your company is from a stage/maturity perspective. If people don't know your company or what you do then your messaging should be very literal, jargon-free. (messaging should be jargon-free generally). If you're more mature and people know what you do then your messaging can be a bit more thought-provoking and inspirational vs. a literal explanation of what you do. 

Here is what I feel:

Show them the trailer to get them to watch the movie.

1. Less is more. Simple and succinct is generally better. 
2. Focus on the most important problem that you want to solve and that keeps it less convoluted.
3. The objective of a hook-worthy statement is to get the prospect or customer to want to find out more. And, 80% right statement that is grounded in the most urgent/recent/important challenge is always better.
4. The copy and where it gets used are important to convey the message. You need to have the messaging 100% right. But your hooks on webpage, social post, etc. can be 80% right and succinct enough to make the prospect/customer move down the funnel or flywheel.