Is it not acceptable to post links in comments? Or is it not acceptable to post links to the same site in comments?
Posting links in comments is not banned, nor should it ever be banned. What should be banned is the repeated posting of a link to an alternative physics Q&A site (such as PO, PF, Quora, etc)1. It is the repetitive nature of the act that is considered as spam,
Electronic spamming is the use of electronic messaging systems to send unsolicited messages (spam), especially advertising, as well as sending messages repeatedly on the same site.
The leaving of the comments acts as a source of free advertising2 for the alternative site while (possibly) detracting users from our site. This is a lose-lose situation for SE and should not be allowed.
Would it be OK if I, say posted links to answers on PO, but also to some on Quora, Wikipedia, etc. which SE considers uncontroversial?
Since Wikipedia is an encyclopedia and not a Q&A site, I have no idea why it is being categorized alongside Quora & PO as some sort of viable alternative to a Q&A site. Links to Wiki are uncontroversial simply because it is not a competitor in any sense of the word3.
Other Q&A sites, such as Quora, Physics Overflow & Physics Forums, are, by their very nature, competitors to the SE network. So if your only actions were posting comments that led users to competitor sites, then yes it should be banned.
Would this also result in a suspension?
That is up to the moderators.
1 I feel it necessary to point out that it was I who noticed the fact that Dilaton's sole action for many days was leaving promotional comments for PO (i.e., spamming) and asked a mod in chat about it; the situation died rather quickly in chat with an inconclusive resolution. The suspension that arose from that comment was not my goal at any point in time, despite that claim to the contrary; it even took me about an hour to catch on what he was getting at.
2 AFAIK, our advertising is free anyways, we simply prefer to have a vote on itsimply prefer to have a vote on it.
3 A similar argument can be made for arXiv that you & Dilaton somehow view as equivalent to Quora, PO & PF, even though it is simply a hosting platform for scientific publications and not in any way a Q&A site.