The 351M/400M engine family has a few bad traits, one of them is the propensity to spin rods bearings. This is due to there being full oil flow to the lifter gallery, it is unnecessary and can cause the valve covers to fill with oil leaving the sump empty.
The well known fix is to put a drilled 1/8" restrictor in the oil feed to the gallery, the gallery itself must be drilled and tapped to accept a pipe plug which has the restriction drilled in. The benefit is less oil in the top end of the engine and an increase in volume to the bottom end. The engine itself is based on the 351 Cleveland and only has a 1" taller deck height, longer stroke and smaller bore (in the 351M) to distinguish it.
The main weakness is the stock rods themselves, they are weak. I would recommend never exceeding 5500 rpm...ever. The good thing is that the heads are interchangeable with the Cleveland and the longer stroke of the Modified family combined with a good set of Cleveland heads and aftermarket rods can make this engine quite strong. I know I have a 400M in my garage tuned to approximately 400HP. They have received a bad rap because of these shortcomings but like anything else they can be overcome with minor component changes. They do make great street engines in modifed form because of the long stroke and make more torque down low as compared to a 351W because of this.
Any of the other problems you mentioned can be attributed to the oiling problem and they disappear when the restrictor is installed. The main reason why they are not all that popular is because a 429/460 is almost a direct swap and most people like to say the word "big block" instead of small block 400. The Lima engines (429/460) and the modified family (351M/400M) have very thin bores except for the earliest versions and anything more than a 0.030" overbore can lead to wall cracking if built to more than 1HP/Cubic inch. Keep these factors in mind when you build and they will provide excellent service.
“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”
— Han Solo