There are good and bad parts of Milwaukee for sure. I'll give my best breakdown. I have lived here pretty much my entire life. I can be wrong but I think I'm close.
Downtown: is pretty obvious and there is very little opportunity to invest. Large multi units, condos, etc.
Lower east side: this is next to downtown and has a lot more opportunity. Young professionals mixed with hipsters and middle aged professionals. Large multi units and a lot of duplexes. Prices are pretty high here. $175k would be inexpensive for a duplex. I don't see a lot of potential for good cash flow and ROI.
Upper east side: near UWM and plenty of pricy student housing. It's mixed with residential. Prices are steep. Low cash flow and ROI IMO.
River west & Brewers hill: are up and coming and have been for quite a while. Can find gems here but a lot of it's been picked through. This is just West of the east side and goes until about Holten. I think it gets rough east of Holten but it's a good line. West of Holten gets bad though.
Low Income area: west of Holten to about 45-50th street north of 94 up to capital is a very low income area. The boards of it have some good gems but in the hart of it is tough.
Wauwatosa: is A-C+ Housing. Just on the North & east border of Wauwatosa is Milwaukee and a spot for some gems. Good rent and affordable price.
West Allis to Waukesha is A-C in my opinion again. Better opportunities inWest Allis but wouldn't limit yourself. This is South of 94 & West of Miller Park Way basically.
3rd to 55th is lower income again. Primarily the Hispanic region. Some good gems in here but some challenges again.
3rd ward, 5th ward: Condos and large multi. New hip area.
Bayview has some gems but has been picked through again. Similar to the upper east and lower east side. $145k would be a steel here.
There are more areas but I'm tired and that is hopefully a start. I have a duplex in the Washington Heights area and am looking at another. Let me know if you'd like any feedback or if I can help in anyway.
If anyone has anything to offer or expand/disagree on 'do like to hear your thoughts.