If the pagebuilder is building on Gutenberg, then there is no lock-in in relation to Gutenberg and the blocks they offer. A client can rest assure, that it is not a niche-pagebuilder I am using - but the future of wordpress. I am sure Bricks knows and will overcome this. I am just afraid and prefer Gutenberg for peace of mind.
I actually have not tested it enough - but Gutenberg does one thing really good - it works on mobile 
For example Breakdance builder can be used without a theme, with a theme - and without the theming-system being active at all - like in Oxygen Builder - oh and also Breakdance has a Gutenberg-block
I dont now anything about the security-issue - but I suspect you are right.
I feel the same about Cwicly, but I am looking at it differently now. I am embarassed to say I bought it twice and got a refund twice. Second time around was to se if the Design library could help me get off the ground running.
Is motion.page necessary for you ? Who is going to pay for it ? Will it be cost-efficient for the client ?
Of course HappyFiles looks amazing - Thomas made it - I mentioned it because you referred to the CMS-aspect of Webflow - which I really like just as you.
A bit about the pros and cons and yes - I know it is a long one - I learned a bit about Bricks in that thread from the people participating → @sunny