I recently got scared by the warnings about continuing to use Office for Mac 2011 and signed up for a trial subscription to Office 365. My wife uses Excel for billing in her business and has about 200 .XLSX files for which print formatting is important. I've found that opening the files with Mac Excel 16.18 results in two types of problem.

The files are opened by default in "normal" view mode, showing the content cells (without the header and footer containing customer information unique to each file). If I attempt to view the header via "view->header & footer", a new, blank header is created and displayed rather than using the contents contained within the .XLSX file. The footer contents remains okay. I've experimented and found that if I open the file and switch view to "page layout", then the original header and footer are displayed and will print as with the earlier version of Excel (14.7.7). Any attempt to view the header while in "normal" display mode results in losing the existing contents. This makes Excel 16.18 unusable.
The existing 200 files have column widths setup to look neat when printed. Opening the files in Mac Excel 16.18 changes the column widths, so that a few items overflow the width and are displayed as "####". I could re-set the column widths in each of the 200 files, but obviously this is not a task I look forward to. I have no idea why the newer version isn't interpreting the settings in these files. Some other minor formatting issues also occur on one row's height and on fill for a row not being displayed in the print copy unless I uncheck "black and white" in the page format dialog
Net result: Office 365 Excel isn't a suitable substitute for Office for Mac 2011 if you have a sizeable amount of formatted files. I don't know what to do - either continue using Excel 2011 or invest a lot of time reformatting files and trying to find a usable header formatting strategy.

Excel will change the number of decimals displayed when a user inputs numbers. For scientific and engineering applications, this has huge impacts on our data.
The input 2.00 is very different than 2, because fewer decimals displayed implies that we have not measured precisely or that we have a large amount of error in the number.
This applies across Excel. For example, I recently made a chart where the data was measured to three significant figures (ie my inputs were 5.24, 6.00, 6.25) but the line graph on the y-axis shows the markers "5.2, 5.6, 6, 6.4." This makes my data look sloppy.
Please, please, please, stop erasing/changing significant figures (decimals) when we input numbers. And please display the number of decimals in data on graphs, in further calculations, and in all other applications.
There is a reason someone chooses to go to the effort of typing "2.000" instead of just "2" and if this causes a problem for some other uses of Excel, there could be an option to disable it.

I have to change the paper size to 90cm width, but obviously it isn't possible anymore in Excel 2016! How am I supposed to print my files now? The support assistant told me that it actually isn't possible, but sent me - as a workaround - a link to a forum entry where people were complaining about the lack of this feature... by the way, the workaround didn't work! ^^ People in the forum suggested using Numbers instead. As a matter of fact, I probably will have to do that as the quality of excel is getting worse and worse through the years...

Whatever the cell width and length are, be clear what it is and give conversion factors to some common units of measure like cm or inches - anything actually measurable with a ruler or micrometer.

You are not even using the same units for vertical and horizontal - Pick one well known unit to use for both directions. Then Help file could give a list of conversions to anyone in any country.

Metric system seems appropriate rather than some unexplained dimensionless number.

Inches is not accepted in my edition of EXCEL contrary to what is described by other users who complain about not using THEIR favorite measure(s).

when opening an excel document in excel (office for mac 2016), made before high sierra, in high sierra, the columns are much smaller and the spacing between characters of a word are much to small and very irregular. So readability gets bad. F.i. arial 8, but also other fonts.

Also underlining is very thick and to close to the characters.

Schermafbeelding 2018-06-16 is how the correct result looked like before going to High Sierra

So I have to seek (but not really finding) another font. Also the column width of all of my documents in excel had to be adapted.

This is not fully visible on screen, but especially shows when printing of making a pdf.

Continuity is a very important quality, but has disappeared!

When the documents are printed in windows, the problems disapear, so they really are linked tot combination with High Sierra.