A poor 404 page and user interaction with it, can only lead to a ‘poor user experience’ signal at Google’s end, for a number of reasons. I will highlight a poor 404 page in my audits and actually programmatically look for signs of this issue when I scan a site. I don’t know if Google looks at your site that way to rate it e.g. algorithmically determines if you have a good 404 page – or if it is a UX factor, something to be taken into consideration further down the line – or purely to get you thinking about 404 pages (in general) to help prevent Google wasting resources indexing crud pages and presenting poor results to searchers. I think rather that any rating would be a second order scoring including data from user activity on the SERPs – stuff we as SEO can’t see.
If you want to *ENSURE* your FULL title tag shows in the desktop UK version of Google SERPs, stick to a shorter title of between 55-65 characters but that does not mean your title tag MUST end at 55 characters and remember your mobile visitors see a longer title (in the UK, in January 2018). What you see displayed in SERPs depends on the characters you use. In 2019 – I just expect what Google displays to change – so I don’t obsess about what Google is doing in terms of display. See the tests later on in this article.
A satisfying UX is can help your rankings, with second-order factors taken into consideration. A poor UX can seriously impact your human-reviewed rating, at least. Google’s punishing algorithms probably class pages as something akin to a poor UX if they meet certain detectable criteria e.g. lack of reputation or old-school SEO stuff like keyword stuffing a site.
Ask for explanations if something is unclear. If an SEO creates deceptive or misleading content on your behalf, such as doorway pages or "throwaway" domains, your site could be removed entirely from Google's index. Ultimately, you are responsible for the actions of any companies you hire, so it's best to be sure you know exactly how they intend to "help" you. If an SEO has FTP access to your server, they should be willing to explain all the changes they are making to your site.
Another illicit practice is to place "doorway" pages loaded with keywords on the client's site somewhere. The SEO promises this will make the page more relevant for more queries. This is inherently false since individual pages are rarely relevant for a wide range of keywords. More insidious, however, is that these doorway pages often contain hidden links to the SEO's other clients as well. Such doorway pages drain away the link popularity of a site and route it to the SEO and its other clients, which may include sites with unsavory or illegal content.
I think ranking in organic listings is a lot about trusted links making trusted pages rank, making trusted links making trusted pages rank ad nauseam for various keywords. Some pages can pass trust to another site; some pages cannot. Some links can. Some cannot. Some links are trusted enough to pass ranking signals to another page. Some are not. YOU NEED LINKS FROM TRUSTED PAGES IF YOU WANT TO RANK AND AVOID PENALTIES & FILTERS.

Ever heard of Maslow's hierarchy of needs? It's a theory of psychology that prioritizes the most fundamental human needs (like air, water, and physical safety) over more advanced needs (like esteem and social belonging). The theory is that you can't achieve the needs at the top without ensuring the more fundamental needs are met first. Love doesn't matter if you don't have food.