Extremely small Investment Banks?!?!?! (The dream or the nightmare)

YMMV greatly. I am at one of the boutiques you describe so I'll give you a data point. My firm is a bit weird in that salaries are well above street but bonuses are well below street at the Analyst level. If you demonstrate that you genuinely want to stay and have a long-term career, then you start getting compensated better.

As Associate, the total comp is street-level to meaningfully-above street level depending on how well the firm does. All cash, none deferred. I'm a VP1 and TC was just shy of $500k after several relatively bad years for the firm from a deal flow perspective (many deals died due to the recent volatility). Average 40 - 55 hours a week so can't really complain about WLB (more like 60 avg. YTD though as activity picks back up).  

It also depends on what kind of clients you serve and what assignments you take on. Boutiques that are pitch-heavy / sell-side focused can be VERY sweaty. Luckily we are more Centerview-like with a retainer model and a few large public clients mainly doing buyside advisory, so punching above our weight means we can survive perfectly fine on retainers alone even when deals are dry. 

Frequently co-advise with a larger BB or EB for deals, which tbh, is a HUGE upside lol. They basically handle all the process work - due diligence, comparing bids, board presentation slide creation, etc. We basically just give a second opinion, aka criticize the shit out of their work since we're on retainer so not incentivized to push bad deals. We'll also do supporting analyses on valuation, pro forma math, investor relations stuff, etc. They get the bigger share of the fee, but with how small we are, we really don't care. 

Average deal size is $500 million - $1 billion+ and our entire firm is ~15 active bankers / ~5 support staff, so a deal or two a year on top of $1 million+ retainers from a handful of key clients is more than enough. We also charge separately for certain special assignments (special committee work, activism defense, etc.)  

Now the downside are (a) we have little "name power" (we're very low key and not often publicized as advisors on press releases), so exiting into MF PE is very tough, though we've had a surprising amount make it into HF, MM PE, etc., (b) zero protection around weekends, vacations, etc. since we're so lean. If a situation comes up, there's literally no one else to cover so have had to cancel multiple vacations, (c) key-man risk. If one or two of our key MDs decides to leave, we're kinda cooked, (d) if you don't like your client, you have basically no way out unless the client decides not to renew because it's an indefinite retainer model as opposed to a defined project scope.