You don't want to strain any relationships with the seller or else you could develop a bad reputation. 

You lose your job, your wife loses her job, the city you invest in gets crushed, the banks you deal with start tightening the reigns, your contractors go under, your funding dries up, etc.

Most of the time mold will not be this type of strain but it depends on geography and length of time left to fester.

I could be wrong but I believe during the housing crash it wasn’t possible to do the BRRRR strategy .In the next downturn I’m sure something similar will happen with lending and banks will tighten up , hard money rates will go up and they won’t be willing to loan as much of the purchase price .

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As money  gets more expensive, it will tighten  up the profit  margins  of investors.

I agree that over-improvement results in a too-long, pay-back period which erodes your returns, but it does not automatically follow that it increases your long-term maintenance and repair costs.You could take an old, draughty, Victorian building, tighten and insulate the building, replace all the windows and doors with more energy efficient fenestration, and improve the HVAC.  

House hacking would be an awesome strategy but many underestimate how much of a headache and strain on a marriage it can be to constantly be dealing with remodeling and tenants.

Also, don’t EVER let sellers pressure you into a crappy financing deal by tightening the timeline.

However, as other people have mentioned, househacking and having neighbors below/above/in the next room will definitely take some getting used to and can strain relationships!